Liquid Crystal Display Device and Electronic Device Including the Same

ABSTRACT

A driver circuit includes first to third transistors, a first circuit, and a second circuit. In the first transistor, a first terminal is electrically connected to a second wiring, a second terminal is electrically connected to a first wiring, and a gate is electrically connected to the second circuit and a first terminal of the third transistor. In the second transistor, a first terminal is electrically connected to the first wiring, a second terminal is electrically connected to a sixth wiring, a gate is electrically connected to the first circuit and a gate of the third transistor. A second terminal of the third transistor is electrically connected to the sixth wiring. The first circuit is electrically connected to a third wiring, a fourth wiring, a fifth wiring, and the sixth wiring. The second circuit is electrically connected to the first wiring, the second wiring, and the sixth wiring.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, a displaydevice, a liquid crystal display device, a driving method thereof, or aproducing method thereof. Specifically, the present invention relates toa semiconductor device, a display device, or a liquid crystal displaydevice including a driver circuit formed over the same substrate as apixel portion, or a driving method of the devices. Alternatively, thepresent invention relates to an electronic device including thesemiconductor device, the display device, or the liquid crystal displaydevice.

2. Description of the Related Art

In recent years, with the increase of large display devices such asliquid crystal televisions, display devices have been activelydeveloped. Specifically, a technique of forming a driver circuit such asa gate driver over the same substrate as a pixel portion by using atransistor formed using a non-single-crystal semiconductor has beenactively developed because the technique makes a great contribution forreduction in cost and improvement in reliability.

Deterioration such as increase in threshold voltage or decrease inmobility is caused in the transistor formed using the non-single-crystalsemiconductor in some cases. As the deterioration of the transistoradvances, there is a problem in that the driver circuit becomes hard tooperate and an image cannot be displayed. Accordingly, Patent Document 1discloses a shift register which can suppress the deterioration of thetransistor. Specifically, in FIG. 7 in Patent Document 1, twotransistors are used for suppression of the deterioration incharacteristics of the transistors. One transistor is connected betweenan output terminal of a flip-flop and a wiring to which VSS (hereinafterreferred to as negative power supply) is supplied. The other transistoris connected between the output terminal of the flip-flop and a gate ofa pull-up transistor. Then, during a period in which an output signalfrom the flip-flop is in an L level, the two transistors are alternatelyturned on. When the one transistor is turned on, VSS is supplied to theoutput terminal of the flip-flop through the one transistor. When theother transistor is turned on, VSS which is supplied to the gate of thepull-up transistor is supplied to the output terminal of the flip-flopthrough the other transistor. In this manner, the deterioration of thetransistor can be suppressed. Further, since VSS is always supplied tothe output terminal of the flip-flop, the output signal from theflip-flop is easily maintained as the L level.

[Reference] [Patent Document] [Patent Document 1] Japanese PublishedPatent Application No. 2005-50502 SUMMARY OF THE INVENTION

In a structure disclosed by Patent Document 1, since the othertransistor is turned on for a while during a period in which the outputsignal from the flip-flop is in an H level, the gate of the pull-uptransistor and the output terminal of the flip-flop are brought intoelectrical conduction for a while. At that time, the gate of the pull-uptransistor has high potential, and the output terminal of the flip-flophas low potential. According to one embodiment of the present invention,it is an object to increase the potential of the gate of the pull-uptransistor.

Alternatively, when the potential of the gate of the pull-up transistoris decreased, the pull-up transistor is turned off in some cases.According to one embodiment of the present invention, it is an object toprevent malfunction of a shift register.

Alternatively, even if the pull-up transistor is turned on and the shiftregister can normally operates, the potential of the gate of the pull-uptransistor is decreased. According to one embodiment of the presentinvention, it is an object to increase a potential difference (Vgs)between the gate and a source of the pull-up transistor.

Alternatively, if Vgs of the pull-up transistor is reduced, theon-resistance of the pull-up transistor is increased. According to oneembodiment of the present invention, it is an object to reduce the sizeof a display device. Alternatively, according to one embodiment of thepresent invention, it is an object to obtain a high-definition displaydevice.

Alternatively, if Vgs of the pull-up transistor is reduced, a risingtime or a falling time of an output signal from the flip-flop becomeslong. According to one embodiment of the present invention, it is anobject to prevent a false signal (e.g., a video signal to a pixel whichbelongs to a different row) from being written to a pixel and to improvedisplay quality.

Alternatively, if Vgs of the pull-up transistor is reduced, the channelwidth of the pull-up transistor needs to be large. In addition, if thechannel width of the pull-up transistor is increased, the channel widthof another transistor also needs to be large. According to oneembodiment of the present invention, it is an object to reduce a layoutarea. Alternatively, according to one embodiment of the presentinvention, it is an object to reduce the size of a frame of a displaydevice.

Alternatively, if the channel width of a transistor is increased, a gateand a source or drain of the transistor are easily short-circuited.According to one embodiment of the present invention, it is an object toimprove yield. Alternatively, according to one embodiment of the presentinvention, it is an object to reduce cost.

Alternatively, if the channel width of the transistor is increased, theparasitic capacitance of the shift register is increased. According toone embodiment of the present invention, it is an object to reducedistortion, delay, or the like of a signal to be input to the shiftregister. Alternatively, according to one embodiment of the presentinvention, it is an object to reduce power consumption. In order tosolve this, as a circuit which supplies a signal, voltage, or the liketo the shift register, a circuit with high current capability needs tobe used. According to one embodiment of the present invention, it is anobject to reduce the size of an external circuit. Alternatively,according to one embodiment of the present invention, it is an object toreduce the size of a display device.

Note that description of the above-described objects does not interruptthe existence of another object.

According to one embodiment of the present invention, a liquid crystaldisplay device includes a driver circuit including a first transistor, asecond transistor, a third transistor, a first circuit, and a secondcircuit, and a pixel including a liquid crystal element. A firstterminal of the first transistor is electrically connected to a secondwiring functioning as a signal line or a clock signal line, a secondterminal of the first transistor is electrically connected to a firstwiring functioning as a signal line, a gate line, a scan line, or anoutput signal line, and a gate of the first transistor is electricallyconnected to the second circuit and a first terminal of the thirdtransistor. A first terminal of the second transistor is electricallyconnected to the first wiring, a second terminal of the secondtransistor is electrically connected to a sixth wiring functioning as apower supply line or a ground line, and a gate of the second transistoris electrically connected the first circuit and a gate of the thirdtransistor. A second terminal of the third transistor is electricallyconnected to the sixth wiring. The second circuit is electricallyconnected to a third wiring functioning as a signal line or a clocksignal line, a fourth wiring functioning as a signal line, a fifthwiring functioning as a signal line, and the sixth wiring. The firstcircuit is electrically connected to the first wiring, the secondwiring, and the sixth wiring.

According to one embodiment of the present invention, the firsttransistor may function as a bootstrap transistor for controlling timingwhen a signal of the second wiring is supplied to the first wiring inaccordance with the potential of the gate of the first transistor.

According to one embodiment of the present invention, the secondtransistor may function as a switch for controlling an electricalconduction state of the sixth wiring and the first wiring in accordancewith an output signal from the first circuit or the potential of thegate of the second transistor.

According to one embodiment of the present invention, the thirdtransistor may function as a switch for controlling an electricalconduction state of the sixth wiring and the gate of the firsttransistor in accordance with an output signal from the first circuit.

According to one embodiment of the present invention, the first circuitmay have a function of increasing, decreasing, and maintaining thepotential of the gate of the second transistor or function as acontrolling circuit for making the gate of the second transistor go intoa floating state by controlling timing when the voltage of the sixthwiring is supplied to the gate of the second transistor in accordancewith a signal from the first wiring or a signal from the second wiring.

According to one embodiment of the present invention, the second circuitmay have a function of increasing, decreasing, and maintaining thepotential of the gate of the first transistor or function as acontrolling circuit for making the gate of the first transistor go intoa floating state by controlling timing when a signal is supplied to thefourth wiring or when the voltage of the sixth wiring is supplied to thegate of the first transistor in accordance with a signal supplied to thethird wiring, a signal supplied to the fourth wiring, or a signalsupplied to the fifth wiring.

According to one embodiment of the present invention, the first circuitincludes a fourth transistor, a fifth transistor, a sixth transistor,and a seventh transistor. A first terminal of the fourth transistor iselectrically connected to the second wiring and a second terminal of thefourth transistor is electrically connected to the gate of the secondtransistor. A first terminal of the fifth transistor is electricallyconnected to the sixth wiring, a second terminal of the fifth transistoris electrically connected to the gate of the second transistor, and agate of the fifth transistor is electrically connected to the firstwiring. A first terminal of the sixth transistor is electricallyconnected to the second wiring, a second terminal of the sixthtransistor is electrically connected to the gate of the fourthtransistor, and a gate of the sixth transistor is electrically connectedto the second wiring. A first terminal of the seventh transistor iselectrically connected to the sixth wiring, a second terminal of theseventh transistor is electrically connected to the gate of the fourthtransistor, and a gate of the seventh transistor is electricallyconnected to the first wiring.

According to one embodiment of the present invention, the second circuitincludes an eighth transistor, a ninth transistor, a tenth transistor,an eleventh transistor, and a twelfth transistor. A first terminal ofthe eighth transistor is electrically connected to the fourth wiring, asecond terminal of the eighth transistor is electrically connected tothe gate of the first transistor, and a gate of the eighth transistor iselectrically connected to the third wiring. A first terminal of theninth transistor is electrically connected to the gate of the firsttransistor, a second terminal of the ninth transistor is electricallyconnected to the fourth wiring, and a gate of the ninth transistor iselectrically connected to the fourth wiring. A first terminal of thetenth transistor is electrically connected to the gate of the firsttransistor, a second terminal of the tenth transistor is electricallyconnected to the sixth wiring, and a gate of the tenth transistor iselectrically connected to the fifth wiring. A first terminal of theeleventh transistor is electrically connected to the first wiring, asecond terminal of the eleventh transistor is electrically connected tothe sixth wiring, and a gate of the eleventh transistor is electricallyconnected to the fifth wiring. A first terminal of the twelfthtransistor is electrically connected to the first wiring, a secondterminal of the twelfth transistor is electrically connected to thesixth wiring, and a gate of the twelfth transistor is electricallyconnected to the third wiring.

According to one embodiment of the present invention, the driver circuitmay be formed over the same substrate as the pixel.

According to one embodiment of the present invention, the channel widthof the first transistor may be larger than that of the second transistorand the third transistor.

Note that a variety of switches can be used as a switch. For example, anelectrical switch, a mechanical switch, or the like can be used. Thatis, any element can be used as long as it can control a current flow,without limitation to a certain element. For example, a transistor(e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PNdiode, a PIN diode, a Schottky diode, an MIM (metal insulator metal)diode, an MIS (metal insulator semiconductor) diode, or adiode-connected transistor), or the like can be used as a switch.Alternatively, a logic circuit in which such elements are combined canbe used as a switch.

An example of a mechanical switch is a switch formed using a MEMS (microelectro mechanical system) technology, such as a digital micromirrordevice (DMD).

Note that a CMOS switch may be used as a switch by using both ann-channel transistor and a p-channel transistor.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, another elementmay be interposed between elements having a connection relationillustrated in drawings and texts, without limitation to a predeterminedconnection relation, for example, the connection relation illustrated inthe drawings and the texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection between A and B(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,and/or a diode) may be connected between A and B. Alternatively, in thecase where A and B are functionally connected, one or more circuitswhich enable functional connection between A and B (e.g., a logiccircuit such as an inverter, a NAND circuit, or a NOR circuit; a signalconverter circuit such as a DA converter circuit, an AD convertercircuit, or a gamma correction circuit; a potential level convertercircuit such as a power supply circuit (e.g., a dc-dc converter, astep-up dc-dc converter, or a step-down dc-dc converter) or a levelshifter circuit for changing a potential level of a signal; a voltagesource; a current source; a switching circuit; an amplifier circuit suchas a circuit which can increase signal amplitude, the amount of current,or the like, an operational amplifier, a differential amplifier circuit,a source follower circuit, or a buffer circuit; a signal generationcircuit; a memory circuit; and/or a control circuit) may be connectedbetween A and B. For example, in the case where a signal output from Ais transmitted to B even when another circuit is interposed between Aand B, A and B are functionally connected.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected with another element or another circuitinterposed therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected withanother circuit interposed therebetween), and the case where A and B aredirectly connected (i.e., the case where A and B are connected withoutanother element or another circuit interposed therebetween) are includedtherein. That is, when it is explicitly described that “A and B areelectrically connected”, the description is the same as the case whereit is explicitly only described that “A and B are connected”.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various modes and can include various elements. Forexample, a display medium, whose contrast, luminance, reflectivity,transmittance, or the like changes by electromagnetic action, such as anEL (electroluminescence) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor which emits light depending on theamount of current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), a digital micromirror device (DMD), apiezoelectric ceramic display, or a carbon nanotube can be used as adisplay element, a display device, a light-emitting element, or alight-emitting device.

A liquid crystal element is an element which controls transmission ornon-transmission of light by an optical modulation action of liquidcrystals and includes a pair of electrodes and liquid crystals. Theoptical modulation action of liquid crystals is controlled by anelectric field applied to the liquid crystal (including a lateralelectric field, a vertical electric field and a diagonal electricfield). Note that the following can be used for a liquid crystalelement: a nematic liquid crystal, a cholesteric liquid crystal, asmectic liquid crystal, a discotic liquid crystal, a thermotropic liquidcrystal, a lyotropic liquid crystal, a low-molecular liquid crystal, ahigh-molecular liquid crystal, a polymer dispersed liquid crystal(PDLC), a ferroelectric liquid crystal, an anti-ferroelectric liquidcrystal, a main-chain liquid crystal, a side-chain high-molecular liquidcrystal, a plasma addressed liquid crystal (PALC), a banana-shapedliquid crystal, and the like. In addition, the following can be used asa diving method of a liquid crystal: a TN (twisted nematic) mode, an STN(super twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS(fringe field switching) mode, an MVA (multi-domain vertical alignment)mode, a PVA (patterned vertical alignment) mode, an ASV (advanced superview) mode, an ASM (axially symmetric aligned microcell) mode, an OCB(optically compensated birefringence) mode, an ECB (electricallycontrolled birefringence) mode, an FLC (ferroelectric liquid crystal)mode, an AFLC (anti-ferroelectric liquid crystal) mode, a PDLC (polymerdispersed liquid crystal) mode, a guest-host mode, a blue phase mode,and the like. Note that the present invention is not limited to this,and various liquid crystal elements and driving methods can be used as aliquid crystal element and a driving method thereof.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source. Note that the presentinvention is not limited to this, and a variety of light sources can beused as a light source.

Note that the structure of a transistor can be a variety of structures,without limitation to a certain structure. For example, a multi-gatestructure having two or more gate electrodes can be used. By using themulti-gate structure, a structure where a plurality of transistors areconnected in series is provided because channel regions are connected inseries.

As another example, a structure where gate electrodes are formed aboveand below a channel can be used.

A structure where a gate electrode is formed above a channel region, astructure where a gate electrode is formed below a channel region, astaggered structure, an inverted staggered structure, a structure wherea channel region is divided into a plurality of regions, or a structurewhere channel regions are connected in parallel or in series can beused. Alternatively, a structure where a source electrode or a drainelectrode overlaps with a channel region (or part of it) can be used.Further, an LDD region may be provided.

Note that when it is explicitly described that “B is formed on A” or “Bis formed over A”, it does not necessarily mean that B is formed indirect contact with A. The description includes the case where A and Bare not in direct contact with each other, i.e., the case where anotherobject is interposed between A and B. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Accordingly, for example, when it is explicitly described that “a layerB is formed on (or over) a layer A”, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or the layer D. Note that another layer (e.g., a layer C or alayer D) may be a single layer or a plurality of layers.

In a similar manner, when it is explicitly described that “B is formedabove A”, it does not necessarily mean that B is formed in directcontact with A, and another object may be interposed therebetween. Thus,for example, when it is described that “a layer B is formed above alayer A”, it includes both the case where the layer B is formed indirect contact with the layer A, and the case where another layer (e.g.,a layer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or the layer D.Note that another layer (e.g., a layer C or a layer D) may be a singlelayer or a plurality of layers.

Note that when it is explicitly described that “B is formed on A”, “B isformed over A”, or “B is formed above A”, it includes the case where Bis formed obliquely over/above A.

Note that the same can be said when it is described that “B is formedbelow A” or “B is formed under A”.

Note that when an object is explicitly described in a singular form, theobject is preferably singular. Note that the present invention is notlimited to this, and the object can be plural. In a similar manner, whenan object is explicitly described in a plural form, the object ispreferably plural. Note that the present invention is not limited tothis, and the object can be singular.

Note that size, the thickness of layers, or regions in diagrams areexaggerated for simplicity in some cases. Therefore, the presentinvention is not necessarily limited to the scale.

Note that diagrams are schematic views of ideal examples, and shapes orvalues are not limited to those illustrated in the diagrams. Forexample, it is possible to include variations in shape due to amanufacturing technique or an error, variations in signals, voltagevalues, or current values due to noise or a difference in timing.

Note that a technical term is used in order to describe a particularembodiment or example or the like in many cases, and is not limited tothis.

Note that terms which are not defined (including terms used for scienceand technology, such as technical terms or academic parlance) can beused as terms which have meaning equal to general meaning that anordinary person skilled in the art understands. It is preferable thatterms defined by dictionaries or the like be construed as consistentmeaning with the background of related art.

Note that terms such as “first”, “second”, “third”, and the like areused for distinguishing various elements, members, regions, layers, andareas from others. Therefore, the terms such as “first”, “second”,“third”, and the like do not limit the number of the elements, members,regions, layers, areas, or the like. Further, for example, “first” canbe replaced with “second”, “third”, or the like.

According to one embodiment of the present invention, the potential of atransistor can be increased. Alternatively, according to one embodimentof the present invention, malfunction can be prevented. Alternatively,according to one embodiment of the present invention, Vgs of atransistor can be increased. Alternatively, according to one embodimentof the present invention, the on-resistance of a transistor can be madelow. Alternatively, according to one embodiment of the presentinvention, the channel width of a transistor can be reduced.Alternatively, according to one embodiment of the present invention, thedeterioration of a transistor can be suppressed or eased. Alternatively,according to one embodiment of the present invention, a layout area canbe reduced. Alternatively, according to one embodiment of the presentinvention, a falling time or a rising time of an output signal of aflip-flop, a shift register, or a driver circuit such as a scan linedriver circuit can be shortened. Alternatively, according to oneembodiment of the present invention, the size of a display device can beincreased. Alternatively, according to one embodiment of the presentinvention, a high-definition display device can be obtained.Alternatively, according to one embodiment of the present invention, aframe of a display device can be narrowed. Alternatively, according toone embodiment of the present invention, a correct signal can be writtento a pixel. Alternatively, according to one embodiment of the presentinvention, display quality can be improved. Alternatively, according toone embodiment of the present invention, yield can be increased.Alternatively, according to one embodiment of the present invention,cost can be reduced. Alternatively, according to one embodiment of thepresent invention, distortion or delay of a signal to be input to ashift register can be reduced. Alternatively, according to oneembodiment of the present invention, power consumption can be reduced.Alternatively, according to one embodiment of the present invention, thecurrent capability of an external circuit can be reduced. Alternatively,according to one embodiment of the present invention, the size of anexternal circuit or the size of a display device including the externalcircuit can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram of a semiconductor device and FIG. 1B is atiming chart illustrating a driving method of the semiconductor device.

FIGS. 2A to 2C are schematic views illustrating a driving method of asemiconductor device.

FIGS. 3A and 3B are schematic views illustrating the driving method ofthe semiconductor device.

FIGS. 4A and 4B are timing charts each illustrating a driving method ofa semiconductor device.

FIGS. 5A and 5B are circuit diagrams of a semiconductor device.

FIGS. 6A to 6C are circuit diagrams of a semiconductor device.

FIGS. 7A to 7C are circuit diagrams of a semiconductor device.

FIG. 8A is a circuit diagram of a semiconductor device and FIG. 8B is atiming chart illustrating a driving method of the semiconductor device.

FIGS. 9A to 9F are schematic views illustrating a driving method of asemiconductor device.

FIGS. 10A to 10F are circuit diagrams of a semiconductor device.

FIG. 11 is a circuit diagram of a semiconductor device.

FIG. 12A is a schematic view illustrating a circuit diagram of asemiconductor device and FIGS. 12B and 12C are schematic viewsillustrating a driving method of the semiconductor device.

FIGS. 13A to 13C are schematic views illustrating the driving method ofthe semiconductor device.

FIGS. 14A to 14C are circuit diagrams of a semiconductor device.

FIGS. 15A and 15B are circuit diagrams of a semiconductor device.

FIGS. 16A and 16B are circuit diagrams of a semiconductor device.

FIGS. 17A to 17C are circuit diagrams of a semiconductor device.

FIGS. 18A and 18B are circuit diagrams of a semiconductor device.

FIG. 19 is a circuit diagram of a shift register.

FIG. 20 is a timing chart illustrating a driving method of a shiftregister.

FIGS. 21A and 21B are timing charts each illustrating a driving methodof a shift register.

FIG. 22 is a circuit diagram of a shift register.

FIGS. 23A and 23B are system block diagrams of a display device.

FIGS. 24A to 24E are diagrams each illustrating a structure of a displaydevice.

FIG. 25A is a circuit diagram of a signal line driver circuit and FIG.25B is a timing chart illustrating a driving method of the signal linedriver circuit.

FIG. 26A is a circuit diagram of a pixel and FIGS. 26B and 26C aretiming charts each illustrating a driving method of the pixel.

FIGS. 27A to 27C are circuit diagrams of a pixel.

FIGS. 28A and 28B are circuit diagrams of a semiconductor device.

FIG. 29A is a top view of a display device and FIGS. 29B and 29C arecross-sectional views of the display device.

FIGS. 30A to 30C are cross-sectional views of a transistor.

FIG. 31 is a layout view of a shift register.

FIG. 32 is a layout view of a shift register.

FIGS. 33A to 33H are diagrams illustrating electronic devices.

FIGS. 34A to 34H are diagrams illustrating electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described withreference to the drawings. However, embodiments can be carried out inmany different modes, and it is easily understood by those skilled inthe art that the mode and detail of the present invention can bemodified in various ways without departing from the spirit and scopethereof. Therefore, the present invention should not be construed asbeing limited to the description of embodiments. Note that in structuresof the present invention described below, reference numerals denotingsimilar components are used in common in different drawings, anddetailed description of the same portions or portions having similarfunctions is omitted.

Note that a content (or may be part of the content) described in oneembodiment may be applied to, combined with, or replaced by a differentcontent (or may be part of the different content) described in the sameembodiment and/or a content (or may be part of the content) described inone or a plurality of different embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with a paragraph disclosed in this specification.

Note that by a combination of a diagram (or may be part of the diagram)described one embodiment with another part of the diagram, a differentdiagram (or may be part of the different diagram) described in the sameembodiment, and/or a diagram (or may be part of the diagram) describedin one or a plurality of different embodiments, much more diagrams canbe formed.

Embodiment 1

In this embodiment, an example of a semiconductor device is described.The semiconductor device in this embodiment can be used for a shiftregister, a gate driver, a source driver, a display device, or the like,for example. Note that the semiconductor device can be referred to as aflip-flop or a driver circuit.

First, an example of the semiconductor device in this embodiment isdescribed with reference to FIG. 1A. A circuit 100 is shown in FIG. 1A.Note that the circuit 100 can be referred to as a semiconductor device,a driver circuit, or a flip-flop.

The circuit 100 includes a transistor 101 (also referred to as a firsttransistor), a transistor 102 (also referred to as a second transistor),a transistor 103 (also referred to as a third transistor), a circuit 104(also referred to as a first circuit), and a circuit 105 (also referredto as a second circuit). The circuit 104 includes a plurality ofterminals: a terminal 104 a, a terminal 104 b, a terminal 104 c, and aterminal 104 d. The circuit 105 includes a plurality of terminals: aterminal 105 a, a terminal 105 b, a terminal 105 c, a terminal 105 d, aterminal 105 e, and a terminal 105 f. However, this embodiment is notlimited to this example. Any of these transistors or any of thesecircuits can be eliminated or replaced with any of a variety of elementssuch as a capacitor, a resistor and a diode, or a circuit in which anyof these elements are combined. Alternatively, a variety of elementssuch as a transistor, a capacitor, a resistor and a diode or a circuitin which any of these elements are combined can be additionallyprovided. Alternatively, a terminal can be added or eliminated dependingon the structure of the circuit 104 and the circuit 105.

Note that for example, the transistors 101 to 103 are n-channeltransistors. The n-channel transistor is turned on when a potentialdifference (Vgs) between a gate and a source of the n-channel transistorexceeds a threshold voltage (Vth). However, this embodiment is notlimited to this example. The transistors 101 to 103 can be p-channeltransistors. The p-channel transistor is turned on when a potentialdifference (Vgs) between a gate and a source of the p-channel transistorbecomes less than a threshold voltage (Vth).

Note that for example, as shown in FIG. 28A, the circuit 104 is atwo-input logic circuit in which an AND circuit and a NOT circuit arecombined. This combinational logic circuit calculates logic conjunctionof one input signal (e.g., a signal of a wiring 113) and an invertedsignal of the other input signal (e.g., a signal of a wiring 111).However, this embodiment is not limited to this example. As the circuit104, a two-input NOR circuit can be used as shown in FIG. 28B.Alternatively, a variety of circuits can be used as the circuit 104.

Note that for example, the circuit 104 and the circuit 105 each includeone or a plurality of transistors. In addition, the polarities of thesetransistors are the same as those of the transistors 101 to 103. Sincethe polarities of the transistors in the circuit 104 and the circuit 105are the same as those of the transistors 101 to 103, the number ofmanufacturing steps can be reduced, yield can be improved, reliabilitycan be improved, and cost can be reduced. However, this embodiment isnot limited to this example. The circuit 104 and the circuit 105 eachcan include an n-channel transistor and a p-channel transistor. That is,the circuit 104 and the circuit 105 can be CMOS circuits.

Note that for example, the terminals 104 a to 104 c function as inputterminals and the terminal 104 d functions as an output terminal. Inaddition, for example, the terminals 105 a to 105 d function as inputterminals and the terminals 105 e and 105 f function as outputterminals. However, this embodiment is not limited to this example.

Note that the circuit 104 and/or the circuit 105 can include even moreterminals. Alternatively, some of the terminals of the circuit 104and/or the circuit 105 can be eliminated.

Next, an example of connection relation in the circuit 100 is described.A first terminal of the transistor 101 is connected to a wiring 112 anda second terminal of the transistor 101 is connected to the wiring 111.A first terminal of the transistor 102 is connected to a wiring 116 anda second terminal of the transistor 102 is connected to the wiring 111.A first terminal of the transistor 103 is connected to the wiring 116, asecond terminal of the transistor 103 is connected to a gate of thetransistor 101, and a gate of the transistor 103 is connected to a gateof the transistor 102. The terminal 104 a of the circuit 104 isconnected to the wiring 112. The terminal 104 b of the circuit 104 isconnected to the wiring 111. The terminal 104 c of the circuit 104 isconnected to the wiring 116. The terminal 104 d of the circuit 104 isconnected to the gate of the transistor 102. The terminal 105 a of thecircuit 105 is connected to the wiring 113. The terminal 105 b of thecircuit 105 is connected to a wiring 114. The terminal 105 c of thecircuit 105 is connected to a wiring 115. The terminal 105 d of thecircuit 105 is connected to the wiring 116. The terminal 105 e of thecircuit 105 is connected to the gate of the transistor 101. The terminal105 f of the circuit 105 is connected to the wiring 111. However, thisembodiment is not limited to this example. A variety of connectionstructures can be employed.

Note that a connection portion of the gate of the transistor 101, thesecond terminal of the transistor 103, or the terminal 105 e of thecircuit 105 is referred to as a node A. In addition, a connectionportion of the gate of the transistor 102, the terminal 104 d of thetransistor 104, or the gate of the transistor 103 is referred to as anode B. Note that the node A and the node B each can be referred to as awiring or a terminal.

Note that the wiring 111, the wiring 112, the wiring 113, the wiring114, the wiring 115, and the wiring 116 each can be referred to as aterminal.

Note that as described above, a terminal can be additionally provided tothe circuit 104 and/or the circuit 105. In that case, the terminal canbe connected to a variety of wirings or a variety of elements.

Note that any of the wirings 111 to 116 can be eliminated and/or awiring can be additionally provided.

Next, examples of a signal or voltage input or output to/from thewirings 111 to 116 are described. For example, a signal OUT is outputfrom the wiring 111. The signal OUT is a digital signal with an H leveland an L level in many cases and can function as an output signal, aselection signal, a transfer signal, a start signal, a reset signal, agate signal, or a scan signal of the circuit 100. For example, a signalIN1 is input to the wiring 112. The signal IN1 is a digital signal inmany cases and can function as a clock signal. For example, a signal IN2is input to the wiring 113. The signal IN2 is out of phase with thesignal IN1 by 180° or an inverted signal of the signal IN1 in many casesand can function as an inverted clock signal. For example, a signal IN3is input to the wiring 114. The signal IN3 is a digital signal in manycases and can function as a start signal or a vertical synchronizationsignal. Alternatively, in the case where the circuit 100 is used for ashift register or a display device, the signal IN3 can function as atransfer signal from a different stage (e.g., the previous stage) or asignal for selecting a different row (e.g., the previous row). Forexample, a signal IN4 is input to the wiring 115. The signal IN4 is adigital signal in many cases and can function as a reset signal.Alternatively, in the case where the circuit 100 is used for the shiftregister or the display device, the signal IN4 can function as a signalfor selecting a different row (e.g., the next row). For example, avoltage V1 is input to the wiring 116. The voltage V1 has approximatelythe same value as the signal OUT in an L level, the signal IN1 in an Llevel, the signal IN2 in an L level, the signal IN3 in an L level, orthe signal IN4 in an L level in many cases and can function as groundvoltage, power supply voltage, or negative power supply voltage.However, this embodiment is not limited to this example. A variety ofsignals, currents, or voltages can be input to the wirings 111 to 116.For example, a voltage such as the voltage V1 or a voltage V2 can besupplied to the wiring 112, the wiring 113, the wiring 114, and/or thewiring 115. Alternatively, a signal such as the signal OUT, the signalIN1, the signal IN2, the signal IN3, or the signal IN4 can be input tothe wiring 116. Alternatively, without input of a signal, voltage, orthe like, the wiring 111, the wiring 112, the wiring 113, the wiring114, the wiring 115, and/or the wiring 116 can be set in a floatingstate.

Note that the term “approximately” includes a variety of errors such asan error due to noise, an error due to variations in process, an errordue to variations in a manufacturing step of an element, and/or an errorof measurement.

Note that the wiring 111 (also referred to as a first wiring) canfunction as a signal line, a gate line, a scan line, or an output signalline. The wiring 112 (also referred to as a second wiring) can functionas a signal line or a clock signal line. The wiring 113 (also referredto as a third wiring) can function as a signal line or a clock signalline. The wiring 114 (also referred to as a fourth wiring) can functionas a signal line. The wiring 115 (also referred to as a fifth wiring)can function as a signal line. The wiring 116 (also referred to as asixth wiring) can function as a power supply line or a ground line.However, this embodiment is not limited to this example. The wirings 111to 116 can function as a variety of wirings. For example, in the casewhere voltage is supplied to the wiring 112, the wiring 113, the wiring114, and/or the wiring 115, these wirings can function as power supplylines. Alternatively, in the case where a signal is input to the wiring116, the wiring 116 can function as a signal line. Alternatively, likethe wiring 111, the wiring 114, and/or the wiring 115 can function as asignal line, a gate line, a scan line, or an output signal line.

Note that polyphase clock signals can be input to the circuit 100. Forexample, in the case of n-phase (n is a natural number) clock signals,the n-phase clock signals are n clock signals whose cycles are differentby 1/n cycle. Alternatively, any two of the polyphase clock signals canbe input to the respective wiring 112 and wiring 113.

Note that as the signal IN1 or the signal IN2, a balanced clock signalor an unbalanced clock signal can be used. The balanced signal is asignal in which a period in which the signal is in an H level is thesame as a period in which the signal is in an L level in one cycle. Theunbalanced signal is a signal in which the period in which the signal isin an H level is different from the period in which the signal is in anL level in one cycle.

Note that for example, the potential of a signal in an L level is V1 andthe potential of a signal in an H level is V2. Further, the V2 is higherthan V1. Furthermore, the term “voltage V2” means that the voltage V2has approximately the same value as the signal in the H level. However,this embodiment is not limited to this example. The potential of thesignal in the L level can be lower or higher than V1. Alternatively, thepotential of the signal in the H level can be lower or higher than V2.

Next, an example of a function of each of the transistors 101 to 103 andthe circuits 104 and 105 is described.

The transistor 101 has a function of controlling timing when the signalOUT goes into an H level by controlling timing when the signal IN1 in anH level is supplied to the wiring 111 in accordance with the potentialof the node A and can function as a pull-up transistor or a bootstraptransistor. The transistor 102 has a function of controlling timing whenthe voltage V1 is supplied to the wiring 111 by controlling anelectrical conduction state of the wiring 116 and the wiring 111 inaccordance with an output signal from the circuit 104 or the potentialof the node B and can function as a switch. The transistor 103 has afunction of controlling timing when the voltage V1 is supplied to thenode A by controlling an electrical conduction state of the wiring 116and the node A in accordance with the output signal from the circuit 104or the potential of the node B and can function as a switch.

The circuit 104 has a function of increasing, decreasing, or maintainingthe potential of the node B or making the node B go into a floatingstate by controlling timing when the signal IN3 or the voltage V1 issupplied to the node B in accordance with the signal OUT or the signalIN1, and can function as a control circuit. Further, the circuit 104 canhave a function of controlling an electrical conduction state of thetransistor 102 and the transistor 103 by controlling the potential ofthe node B. For example, the circuit 104 has a function of decreasingthe potential of the node B by supplying the voltage V1 or the signalIN2 in an L level to the node B when the signal IN2 goes into the Llevel. As another example, the circuit 104 has a function of decreasingthe potential of the node B by supplying the voltage V1 or a signal inan L level to the node B when the signal OUT goes into an H level. Asanother example, the circuit 104 has a function of increasing thepotential of the node B by supplying the voltage V2 or the signal IN2 inan H level to the node B when the signal IN2 goes into the H level inthe case where the signal OUT is in an L level.

The circuit 105 has a function of increasing, decreasing, or maintainingthe potential of the node A or making the node A go into a floatingstate by controlling timing when the signal IN3 or the voltage V1 issupplied to the node A in accordance with the signal IN2, the signalIN3, or the signal IN4, and can function as a control circuit.Alternatively, the circuit 105 has a function of decreasing ormaintaining the potential of the wiring 111 or making the wiring 111 gointo a floating state by controlling timing when the voltage V1 issupplied to the wiring 111 in accordance with the signal IN2, the signalIN3, or the signal IN4. For example, the circuit 105 has a function ofincreasing the potential of the node A by supplying the signal IN3 in anH level or the voltage V2 to the node A when the signal IN2 or thesignal IN3 goes into the H level. As another example, the circuit 105has a function of decreasing the potential of the node A or the wiring111 by supplying the voltage V1 or a signal in an L level to the node Aor the wiring 111 when the signal IN2 or the signal IN4 goes into an Hlevel.

However, this embodiment is not limited to this example. The transistors101 to 103 and the circuits 104 and 105 can have a variety of functions.Alternatively, it is acceptable that these elements or circuits do nothave the above-described functions.

Next, operation of the semiconductor device in FIG. 1A is described withreference to FIG. 1B, FIGS. 2A to 2C, and FIGS. 3A and 3B. FIG. 1B is anexample of a timing chart for describing the operation of thesemiconductor device. FIG. 1B shows an example of each of the signalIN1, the signal IN2, the signal IN3, the signal IN4, a potential Va ofthe node A, a potential Vb of the node B, and the signal OUT in oneoperation period. In addition, one operation period in the timing chartin FIG. 1B includes a period T1, a period T2, a period T3, periods T4,and periods T5. FIG. 2A shows an example of a schematic view of theoperation of the semiconductor device in FIG. 1A during the period T1.FIG. 2B shows an example of a schematic view of the operation of thesemiconductor device in FIG. 1A during the period T2. FIG. 2C shows anexample of a schematic view of the operation of the semiconductor devicein FIG. 1A during the period T3. FIG. 3A shows an example of a schematicview of the operation of the semiconductor device in FIG. 1A during theperiod T4. FIG. 3B shows an example of a schematic view of the operationof the semiconductor device in FIG. 1A during the period T5.

Note that for example, when the signal IN3 goes into an H level, thesemiconductor device in FIG. 1A sequentially performs operation of theperiod T1, operation of the period T2, and operation of the period T3.Then, after that, the semiconductor device in FIG. 1A repeatsalternately operation of the period T4 and the operation of the periodT5 until the signal IN3 goes into the H level again. However, thisembodiment is not limited to this example. The semiconductor device inFIG. 1A can perform the operations of the periods T1 to T5 in a varietyof orders.

First, in the period T1, the signal IN1 goes into an L level, the signalIN2 goes into an H level, the signal IN3 goes into an H level, and thesignal IN4 goes into an L level. Since the signal IN3 goes into the Hlevel, the circuit 105 starts to raise the potential of the node A. Atthat time, since the signal IN1 goes into the L level, the circuit 104starts to decrease the potential of the node B to V1. Accordingly, sincethe transistor 102 and the transistor 103 are turned off, the wiring 116and the wiring 111 are brought out of electrical conduction, whereby thewiring 116 and the node A are brought out of electrical conduction.After that, when the potential of the node A becomes equal to the sum ofthe potential of the wiring 112 (V1) and the threshold voltage of thetransistor 101 (Vth101), (V1+Vth101), the transistor 101 is turned on.Then, since the wiring 112 and the wiring 111 are brought intoelectrical conduction, the signal IN1 in the L level is supplied to thewiring 111 from the wiring 112 through the transistor 101. Accordingly,since the potential of the wiring 111 becomes V1, the signal OUT goesinto an L level. After that, the circuit 105 continues to further raisethe potential of the node A. In addition, the circuit 105 stops supplyof a signal or voltage to the node A when the potential of the node A israised to a certain value (at least greater than or equal to V1+Vth101).Accordingly, the node A goes into a floating state while maintaining thepotential at that time (e.g., greater than or equal to V1+Vth101).

Note that in the period T1, the circuit 105 supplies the voltage V1 or asignal in an L level to the wiring 111 in many cases. However, thisembodiment is not limited to this example. If the circuit 105 does notsupply the voltage, signal, or the like to the wiring 111, the circuit105 and the wiring 111 can be brought out of electrical conduction.

Next, in the period T2, the signal IN1 goes into an H level, the signalIN2 goes into an L level, the signal IN3 goes into an L level and thesignal IN4 is kept as the L level. Since the circuit 105 does not supplyvoltage, signal, or the like to the node A in many cases, the node A iskept in the floating state while maintaining the potential in the periodT1 (greater than or equal to V1+Vth101). Accordingly, since thetransistor 101 is kept on, the wiring 112 and the wiring 111 are kept inelectrical conduction. At that time, since the level of the signal IN1is raised to an H level from the L level, the potential of the wiring111 starts to rise from V1. Then, since the node A is in the floatingstate, the potential of the node A is raised by the parasiticcapacitance between the gate and the second terminal of the transistor101; so-called bootstrap operation is performed. In this manner, thepotential of the node A is raised to (V2+Vth101+α) (α is a positivenumber). Then, the potential of the wiring 111 rises to the potential ofthe signal IN2 in the H level, that is, V2, whereby the signal OUT goesinto an H level. At that time, since the signal OUT goes into the Hlevel, the circuit 104 maintains the potential of the node B as V1 bysupplying the voltage V1 or a signal in an L level to the node B.Accordingly, since the transistor 102 and the transistor 103 are keptoff, the wiring 116 and the wiring 111 are kept out of electricalconduction, whereby the wiring 116 and the node A are kept out ofelectrical conduction.

Note that in the period T2, if the circuit 104 does not supply thesignal, voltage, or the like to the node B, the circuit 104 and the nodeB can be brought out of electrical conduction. Then, the circuit 104 canmake the node B go into a floating state. In that case too, since thenode B goes into the floating state, the potential of the node B ismaintained as V1 in many cases.

Note that if the circuit 105 does not supply the voltage, signal, or thelike to the wiring 111 in the period T2, the circuit 105 and the wiring111 can be brought out of electrical conduction. However, thisembodiment is not limited to this example. The circuit 105 can supplythe voltage V2, a signal in an H level, or the like to the wiring 111.

Next, in the period T3, the signal IN1 goes into the L level, the signalIN2 goes into the H level, the signal IN3 is kept at the L level, andthe signal IN4 goes into an H level. Since the signal IN4 goes into theH level, the circuit 105 decreases the potential of the node A to V1.Therefore, since the transistor 101 is turned off, the wiring 112 andthe wiring 111 are brought out of electrical conduction. Here, since thepotential of the node A is controlled by voltage or a signal suppliedthrough the circuit 105, timing when the transistor 101 is turned offcomes later than timing when the signal IN1 goes into the L level inmany cases. That is, when the transistor 101 is on, the signal IN1 goesinto the L level in some cases. In that case, the signal IN1 in the Llevel is supplied to the wiring 111 from the wiring 112 through thetransistor 101. Accordingly, since the potential of the wiring 111 isV1, the signal OUT goes into the L level. At that time, since the signalIN1 is in the L level, the circuit 104 keeps the potential of the node Bas V1 by supplying the signal IN2 in the L level or the voltage V1 tothe node B. Accordingly, since the transistor 102 and the transistor 103are kept off, the wiring 116 and the wiring 111 are kept out ofelectrical conduction, whereby the wiring 116 and the node A are keptout of electrical conduction.

Note that in the period T3, if the circuit 104 does not supply thesignal, voltage, or the like to the node B, the circuit 104 and the nodeB can be brought out of electrical conduction. Then, the circuit 104 canmake the node B go into a floating state. In that case too, since thenode B goes into the floating state, the potential of the node B ismaintained as V1 in many cases.

Note that in the period T2, the circuit 105 can supply the voltage V1 ora signal in an L level to the wiring 111. Alternatively, if the circuit105 does not supply the voltage, signal, or the like to the wiring 111,the circuit 105 and the wiring 111 can be brought out of electricalconduction.

Next, in the period T4, the signal IN1 goes into the H level, the signalIN2 goes into the L level, the signal IN3 is kept at the L level, andthe signal IN4 goes into the L level. Since the signal OUT is kept atthe L level while the signal IN1 goes into the H level, by supplying thesignal IN1 in the H level or the voltage V2 to the node B, the circuit104 raises the potential of the node B to V2. Then, since the transistor102 and the transistor 103 are turned on, the wiring 116 and the wiring111 are brought into electrical conduction, whereby the wiring 116 andthe node A are brought into electrical conduction. Accordingly, sincethe voltage V1 is supplied to the wiring 111 from the wiring 116 throughthe transistor 102, the potential of the wiring 111 is maintained as V1.Then, since the voltage V1 is supplied to the node A from the wiring 116through the transistor 103, the potential of the node A is maintained asV1. In this manner, the signal OUT is kept at the L level.

Note that the circuit 105 can supply the voltage V1, a signal in an Llevel, or the like to the wiring 111 or the node A. Alternatively, ifthe circuit 105 does not supply the voltage, signal, or the like to thewiring 111 or the node A, the circuit 105 and the node A can be broughtout of electrical conduction and the circuit 105 and the wiring 111 canbe brought out of electrical conduction.

Next, in the period T5, the signal IN1 goes into the L level, the signalIN2 goes into the H level, the signal IN3 is kept at the L level, andthe signal IN4 is kept at the L level. Since the signal IN1 goes intothe L level, the circuit 104 decreases the potential of the node B to V1by supplying the signal IN1 in the L level or the voltage V1 to the nodeB. Accordingly, since the transistor 102 and the transistor 103 areturned off, the wiring 116 and the wiring 111 are brought out ofelectrical conduction, whereby the wiring 116 and the node A are broughtout of electrical conduction. Here, if the circuit 105 supplies thevoltage V1, a signal in an L level, or the like to the wiring 111 or thenode A, the potential of the wiring 111 or the node A is maintained asV1. However, even in the case where the circuit 105 does not supply thevoltage, signal, or the like to the wiring 111 or the node A, thepotential of the wiring 111 or the node A is maintained as V1. This isbecause, since the wiring 111 and the node A go into a floating state,the potential in the period T4 (V1) is maintained. In this manner, thesignal OUT is kept at the L level.

The above is the description of the operation of the semiconductordevice in FIG. 1A. In the semiconductor device in FIG. 1A, decrease inthe potential of the node A can be prevented in the period T2. In aconventional technique, the node A and the wiring 111 are in electricalconduction until the potential of the wiring 111 is raised to a certainvalue in the period T2. Accordingly, the potential of the node A isdecreased. However, in the semiconductor device in FIG. 1A, the node Aand the wiring 111 are not brought into electrical conduction in theperiod T2. Therefore, decrease in the potential of the node A can beprevented. As a result, decrease in Vgs of the transistor 101 can beprevented. Alternatively, Vgs of the transistor 101 can be increased.Alternatively, malfunction due to too much decrease in the potential ofthe node A can be prevented. Alternatively, since the decrease in Vgs ofthe transistor 101 can be prevented, the channel width (W) of thetransistor 101 can be reduced. Accordingly, reduction in a layout areacan be achieved. Alternatively, since Vgs of the transistor 101 can beincreased, the on-resistance of the transistor 101 can be reduced.Therefore, a falling time or a rising time of the signal OUT can beshortened or delay of the signal OUT can be reduced.

Alternatively, all the transistors in the semiconductor device in FIG.1A can be n-channel transistors or all the transistors in thesemiconductor device in FIG. 1A can be p-channel transistors.Accordingly, reduction in the number of steps, improvement in yield,improvement in reliability, or reduction in cost can be achieved.Specifically, if all the transistors are n-channel transistors,non-single-crystal semiconductors, microcrystalline semiconductors,organic semiconductors, oxide semiconductors, or the like can be usedfor semiconductor layers of the transistors. Accordingly, reduction inthe number of steps, improvement in yield, improvement in reliability,or reduction in cost can be achieved. However, this embodiment is notlimited to this example. The semiconductor device in FIG. 1A can includea CMOS circuit which includes a p-channel transistor and an n-channeltransistor. Moreover, single crystal semiconductors or polycrystallinesemiconductors can be used for the semiconductor layers of thetransistors.

Alternatively, in the semiconductor device in FIG. 1A, the transistors101 to 103 are turned off in at least one of the period T4 and theperiod T5. Therefore, since the transistor stays in an off state duringone operation period, deterioration of characteristics of thetransistor, such as increase in threshold voltage or decrease inmobility, can be suppressed. Specifically, if a non-single-crystalsemiconductor, a microcrystalline semiconductor, an organicsemiconductor, an oxide semiconductor, or the like is used for asemiconductor layer of the transistor, the characteristics of thetransistor obviously deteriorates in many cases. However, in thesemiconductor device in FIG. 1A, a non-single-crystal semiconductor, amicrocrystalline semiconductor, an organic semiconductor, an oxidesemiconductor, or the like can be easily used for the semiconductorlayer of the transistor because deterioration of characteristics of thetransistor can be suppressed. However, this embodiment is not limited tothis example. A polycrystalline semiconductor or a single crystalsemiconductor can be used for the semiconductor layer.

Note that the period T2 can be referred to as a selection period and theother periods (the period T1, the period T3, the period T4, and theperiod T5) can be referred to as non-selection periods. Alternatively,the period T1, the period T2, the period T3, the period T4, and theperiod T5 can be referred to as a set period, an output period, a resetperiod, a first non-selection period, and a second non-selection period,respectively.

Note that although one example of the timing chart in FIG. 1B shows thecase where the signal IN1 and the signal IN2 are balanced signals, thisembodiment is not limited to this example. As described above, thesignal IN1 and the signal IN2 can be unbalanced signals. Alternatively,although the timing chart in FIG. 1B shows the case where a period inwhich the signal IN1 (or the signal IN2) is in the H level and a periodin which the signal IN1 (or the signal IN2) is in the L level areapproximately equal, that is, the case where the duty ratio of thesignal IN1 and the signal IN2 is approximately 50%, this embodiment isnot limited to this example. The duty ratio of the signal IN1 and thesignal IN2 can be greater than or equal to 50% or less than or equal to50%. FIG. 4A shows a timing chart in the case where the signal IN1 andthe signal IN2 are unbalanced signals and the duty ratio of the signalIN1 and the signal IN2 is not 50%. In the timing chart in FIG. 4A, inthe period T2, when the signal IN1 goes into an H level, the potentialof the node A is raised by bootstrap operation and the signal OUT goesinto an H level. After that, the signal IN1 goes into an L level. In thetiming chart in FIG. 1B, the potential of the node A is decreased to V1at the same time as or little later than the time when the signal IN1goes into the L level. That is, the transistor 101 is turned off at thesame time as or little later than the time when the signal IN1 goes intothe L level. However, in the timing chart in FIG. 4A, the potential ofthe node A is kept high until the signal IN4 goes into an H level or thesignal IN2 goes into an H level. That is, even after the signal IN1 goesinto the L level, the transistor 101 is kept on. Accordingly, since thewiring 112 and the wiring 111 are kept in electrical conduction, thesignal IN1 in the L level is supplied to the wiring 111 from the wiring112 through the transistor 101. Accordingly, since the channel width (W)of the transistor 101 is large in many cases, the potential of thewiring 111 is immediately decreased to V1. Therefore, a falling time ofthe signal OUT can be shortened.

Note that in FIG. 4A, one cycle of the signal IN is referred to as aperiod Tck. In addition, a period in which the signal IN1 is in an Hlevel in one cycle is referred to as a period Tck(H) and a period inwhich the signal IN1 is in an L level in one cycle is referred to as aperiod Tck(L). Similarly, one cycle of the signal IN2 is referred to asa period Tckb. In addition, a period in which the signal IN2 is in the Hlevel in one cycle is referred to as a period Tckb(H) and a period inwhich the signal IN2 is in the L level in one cycle is referred to as aperiod Tckb(L). The relation between the period Tck and the period Tckbis shown by the formula, Tck≈Tckb in many cases. The relation betweenthe period Tck(H) and the period Tckb(H) is shown by the formula,Tck(H)≈Tckb(H) in many cases. The relation between the period Tck(L) andthe Tckb(L) is shown by the formula, Tck(L)≈Tckb(L) in many cases.However, this embodiment is not limited to this example.

Note that the relation between the period Tck(H) and the period Tck(L)is preferably shown by the formula, Tck(H)<Tck(L) in FIG. 4A. Similarly,the relation between the period Tckb(H) and the period Tckb(L) ispreferably shown by the formula, Tckb(H)<Tckb(L). In this manner, asdescribed above, the falling time of the signal OUT can be shortened.However, this embodiment is not limited to this example. The relationbetween the period Tck(H) and the period Tck(L) can be shown by theformula, Tck(H)>Tck(L) and the relation between the period Tckb(H) andthe period Tckb(L) can be shown by the formula, Tckb(H)>Tckb(L).

Note that as shown by the timing chart in FIG. 4A, the signal OUT can gointo an L level in the course of the period T2. In order to achievethis, the signal IN4 goes into an H level in the course of the periodT2. Then, the circuit 100 in FIG. 1A is forced to start operation in theperiod T3 or operation which is substantially the same as the operationin the period T3. First, since the signal IN4 goes into the H level, thecircuit 105 decreases the potential of the node A and the wiring 111 toV1 by supplying the V1 or a signal in an L level to the node A and thewiring 111. Accordingly, the signal OUT goes into the L level. Inaddition, since the signal OUT is in the L level and the signal IN1 iskept at the H level, the circuit 104 makes the potential of the node BV2 by supplying the signal IN1 in the H level to the node B like in theperiod T4. Accordingly, since the transistor 102 and the transistor 103are turned on, the wiring 116 and the wiring 111 are brought intoelectrical conduction and the wiring 116 and the node A are brought intoelectrical conduction. Therefore, since the voltage V1 is supplied tothe wiring 111 from the wiring 116 through the transistor 102, thepotential of the wiring 111 is maintained as V1. On the other hand,since the voltage V1 is supplied to the node A from the wiring 116through the transistor 103, the potential of the node A is maintained asV1. At that time, since the potential of the node A is V1, thetransistor 101 is turned off. Therefore, the wiring 112 and the wiring111 are brought out of electrical conduction. In this manner, the periodin which the signal OUT is in the H level can be shorter than the periodin which the signal IN1 is in the H level. As a result, the drivingfrequency becomes low as compared to the case where the period in whichthe signal IN1 is in the H level is approximately equal to the period inwhich the signal OUT is in the H level. Therefore, power consumption canbe reduced.

Note that for example, the transistor 101 preferably has the largestchannel width among the transistors 101 to 103 or the transistorsincluded in the semiconductor device in FIG. 1A. In this manner, theon-resistance of the transistor 101 is reduced, whereby the rising timeor the falling time of the signal OUT can be shortened. However, thisembodiment is not limited to this example. The channel width of thetransistor 101 can be smaller than that of any of the transistorsincluded in the semiconductor device in FIG. 1A.

Note that the term “the channel width of a transistor” can also bereferred to as the W/L (W is channel width and L is channel length)ratio of a transistor.

Note that for example, the channel width of the transistor 102 ispreferably larger than that of the transistor 103. This is because,since the wiring 111 is connected to a gate line, a pixel, or the likein many cases, the load of the wiring 111 is heavier than that of thenode A in many cases. In addition, another reason is that the transistor102 has a function of supplying the voltage V1 to the wiring 111 and thetransistor 103 has a function of supplying the voltage V1 to the node A.However, this embodiment is not limited to this example. The channelwidth of the transistor 102 can be smaller than that of the transistor103.

Note that for example, the parasitic capacitance between the gate andthe second terminal of the transistor 101 is preferably higher than theparasitic capacitance between the gate and the first terminal of thetransistor 101. This is because the potential of the node A is easilyincreased by bootstrap operation in the period T2. Therefore, an areawhere a conductive layer which functions as a gate and a conductivelayer which functions as a source or drain on the second terminal sideis preferably larger than that on the first terminal side. However, thisembodiment is not limited to this example.

Note that the wiring can be divided into a plurality of wirings. Inaddition, the same signal or voltage or different signals or voltagescan be input to the plurality of wirings. Alternatively, the pluralityof wirings can be connected to the same wiring or element or differentwirings or elements. One example in FIG. 5A illustrates a structure inthe case where the wiring 112 is divided into a plurality of wirings ofwirings 112A and 112B and the wiring 116 is divided into a plurality ofwirings of wirings 116A to 116D. The first terminal of the transistor101 is connected to the wiring 112A and the terminal 104 a of thecircuit 104 is connected to the wiring 112B. The first terminal of thetransistor 102 is connected to the wiring 116A and the first terminal ofthe transistor 103 is connected to the wiring 116B. The terminal 104 cof the circuit 104 is connected to the wiring 116C and the terminal 105d of the circuit 105 is connected to the wiring 116D. However, thisembodiment is not limited to this example. The wiring 111, the wiring113, the wiring 114, and/or the wiring 115 each can be divided into aplurality of wirings. Alternatively, only one of the wiring 112 and thewiring 116 can be divided into a plurality of wirings.

Note that in FIG. 5A, the wirings 112A and 112B correspond to the wiring112 in FIG. 1A. Therefore, the signal IN1 can be input to the wirings112A and 112B and the wirings 112A and 112B can function as signal linesor clock signal lines. However, this embodiment is not limited to thisexample. Voltages such as the voltage V1 and the voltage V2 can besupplied to the wirings 112A and 112B and the wirings 112A and 112B canfunction as power supply lines. Alternatively, different signals orvoltages can be input to the wirings 112A and 112B. Alternatively, otherthan above, a variety of signals, voltages, or currents can be input tothe wirings 112A and 112B.

Note that in FIG. 5A, the wirings 116A to 116D correspond to the wiring116 in FIG. 1A. Therefore, the voltage V1 can be supplied to the wirings116A to 116D and the wirings 116A to 116D can function as power supplylines. However, this embodiment is not limited to this example. A signalsuch as the signal OUT, the signal IN1, the signal IN2, the signal IN3,or the signal IN4 can be input to the wirings 116A to 116D and thewirings 116A to 116D can function as signal lines. Alternatively,different signals or voltages can be input to the wirings 116A to 116D.Alternatively, other than above, a variety of signals, voltages, orcurrents can be input to the wirings 116A to 116D.

Note that in FIG. 5A, a signal in an L level in the period T4 can beinput to the wirings 116A and 116B. For example, the signal IN2 can beinput to the wirings 116A and 116B. In that case, as shown in FIG. 5B,the first terminal of the transistor 102 and the first terminal of thetransistor 103 can be connected to the wiring 113. In this manner, sincebackward bias can be applied to the transistor 102 and the transistor103, deterioration of characteristics of the transistor 102 and thetransistor 103 can be eased. However, this embodiment is not limited tothis example. The signal IN2 can be input to one of the wirings 116A and116B and only one of the wirings 116A and 116B can be connected to thewiring 113. Alternatively, the signal OUT, the signal IN3, the signalIN4, or another signal can be input to the wiring 116A and/or the wiring116B. In that case, the first terminal of the transistor 103 and/or thefirst terminal of the transistor 102 can be connected to the wiring 111,the wiring 114, or the wiring 115. Alternatively, the signal OUT, thesignal IN2, the signal IN3, the signal IN4, or another signal can beinput to the wiring 116C and/or the wiring 116D. In that case, theterminal 104 c of the circuit 104 and/or the terminal 105 d of thecircuit 105 can be connected to the wiring 111, the wiring 113, thewiring 114, or the wiring 115.

Note that as shown in FIG. 6A, a capacitor 121 can be additionallyconnected between the gate and the second terminal of the transistor101. In this manner, the potential of the node A can be made high in thebootstrap operation in the period T2. Accordingly, since Vgs of thetransistor 101 is increased, the falling time or rising time of thesignal OUT can be shortened. However, this embodiment is not limited tothis example. A transistor can be used as a MOS capacitor of thecapacitor 121. In that case, in order to increase the capacitance valueof the transistor used as the MOS capacitor, it is preferable that thegate of the transistor be connected to the node A and the first terminalor the second terminal of the transistor be connected to the wiring 111.

Note that like in FIG. 6A, the capacitor 121 can be additionallyconnected between the gate and the second terminal of the transistor 101in FIGS. 5A and 5B. Alternatively, a transistor whose first terminal andsecond terminal are connected to the wiring 111 and whose gate isconnected to the node A can be additionally provided.

Note that as shown in FIG. 6B, a transistor 122 whose first terminal isconnected to the wiring 111, whose second terminal is connected to thenode A, and whose gate is connected to the wiring 112 can beadditionally provided. The transistor 122 preferably has the samepolarity as the transistors 101 to 103 and is an n-channel transistor inmany cases. However, this embodiment is not limited to this example. Thetransistor 122 can be a p-channel transistor. The transistor 122 has afunction of controlling timing when the node A and the wiring 111 arebrought into electrical conduction in accordance with the signal IN2 andcan function as a switch. The transistor 122 is turned on in the periodT4 and brings the node A and the wiring 111 into electrical conduction.

Note that as shown in FIG. 6B, the transistor 122 whose first terminalis connected to the wiring 111, whose second terminal is connected tothe node A, and whose gate is connected to the wiring 112 can beadditionally provided in FIGS. 5A and 5B and FIG. 6A.

Note that as shown in FIG. 6C, the transistor 103 can be eliminated. Inthat case, the node A is in a floating state in the period T4 in manycases. However, this embodiment is not limited to this example. Thetransistor 102 can be eliminated. In that case, the wiring 111 is in afloating state in the period T4 in many cases. In this manner, byelimination of one of the transistor 102 and the transistor 103, thenumber of transistors can be reduced. Therefore, reduction in a layoutarea, improvement in yield, or the like can be achieved.

Note that like in FIG. 6C, the transistor 102 or the transistor 103 canbe eliminated in FIGS. 5A and 5B and FIGS. 6A and 6B. Specifically inFIG. 6B, one of the transistor 102 and the transistor 103 is preferablyeliminated. This is because, since the node A and the wiring 111 arebrought into electrical conduction in the period T4, the node A or thewiring 111 does not go into a floating state.

Note that as shown in FIG. 7A, the transistor 102 can be replaced with adiode 102 a one terminal (also referred to as a positive electrode) ofwhich is connected to the wiring 111 and the other terminal (alsoreferred to as a negative electrode) of which is connected to the nodeB. In addition, the transistor 103 can be replaced with a diode 103 aone terminal (also referred to as a positive electrode) of which isconnected to the node A and the other terminal (also referred to as anegative electrode) of which is connected to the node B. In that case,the circuit 104 can decrease the potential of the node B to V1 in theperiod T4 and increase the potential of the node B to V2 in the periodT1, the period T2, and the period T5. However, this embodiment is notlimited to this example. Only one of the transistor 102 and thetransistor 103 can be replaced with a diode. Alternatively, the diode102 a and/or the diode 103 a can be additionally provided.

Note that like in FIG. 7A, the transistor 102 can be replaced with thediode 102 a one terminal of which is connected to the wiring 111 and theother terminal of which is connected to the node B in FIGS. 5A and 5Band FIGS. 6A to 6C. Alternatively, the transistor 103 can be replacedwith the diode 103 a one terminal of which is connected to the node Aand the other terminal of which is connected to the node B.Alternatively, the diode 102 a and/or the diode 103 a can beadditionally provided.

Note that although not shown, the transistor 102 or the transistor 103can be diode-connected in FIG. 1A, FIGS. 5A and 5B, FIGS. 6A to 6C, andFIG. 7A. In that case, the first terminal of the transistor 102 isconnected to the node B, the second terminal of the transistor 102 isconnected to the wiring 111, and the gate of the transistor 102 isconnected to the node B or the wiring 111. The first terminal of thetransistor 103 is connected to the node B, the second terminal of thetransistor 103 is connected to the node A, and the gate of thetransistor 103 is connected to the node A or the node B. However, thisembodiment is not limited to this example. Only one of the transistor102 and the transistor 103 can be diode-connected.

Note that as shown in FIG. 7B, the terminal 104 b of the circuit 104 canbe connected to the node A. In this manner, since a signal in an L levelcan be prevented from being input to the terminal 104 b of the circuit104 in the period T2, the potential of the node B is easily maintainedas V1. Therefore, the transistor 102 and the transistor 103 can beprevented from being turned on because of an instant increase in thepotential of the node B.

Note that like in FIG. 7B, the terminal 104 b of the circuit 104 can beconnected to the node A in FIGS. 5A and 5B, FIG. 6A to 6C, and FIG. 7A.

Note that as shown in FIG. 7C, the circuit 105 can be eliminated.

Note that like in FIG. 7C, the circuit 105 can be eliminated in FIGS. 5Aand 5B, FIG. 6A to 6C, and FIGS. 7A and 7B.

Note that as shown in FIG. 28B, the terminal 104 a of the circuit 104can be connected to the wiring 113. However, this embodiment is notlimited to this example. The terminal 104 a of the circuit 104 can beconnected to a variety of wirings, terminals, or nodes. Note that likein FIG. 28B, the terminal 104 a of the circuit 104 can be connected tothe wiring 113 in FIGS. 5A and 5B, FIGS. 6A to 6C, and FIGS. 7A and 7B.

Note that as shown in FIG. 8A, p-channel transistors can be used as thetransistors 101 to 103. A transistor 101 p, a transistor 102 p, and atransistor 103 p corresponds to the transistor 101, the transistor 102,and the transistor 103, respectively, and are p-channel transistors. Inaddition, as shown in FIG. 8B, in the case where transistors arep-channel transistors, the voltage V2 is supplied to the wiring 116 andthe signal OUT, the signal IN1, the signal IN2, the signal IN3, thesignal IN4, the potential of the node A, and the potential of the node Bare inverted signals and potentials of those in the timing chart in FIG.1B.

Note that in FIG. 8A, the transistors included in the circuit 104 andthe circuit 105 are preferably p-channel transistors. However, thisembodiment is not limited to this example. The transistors included inthe circuit 104 and the circuit 105 can be n-channel transistors.

Note that like in FIGS. 8A and 8B, p-channel transistors can be used asthe transistors 101 to 103 in FIGS. 5A and 5B, FIGS. 6A to 6C, and FIGS.7A to 7C.

Embodiment 2

In this embodiment, a specific example of the circuit 104 described inEmbodiment 1 is described. Note that the circuit 104 can be referred toas a semiconductor device, a driver circuit, or a gate driver. Note thatdescription of the content described in Embodiment 1 is omitted. Notethat the content described in Embodiment 1 can be freely combined with acontent described in this embodiment.

First, one example of the circuit 104 is described with reference toFIG. 9A. In the example in FIG. 9A, the circuit 104 includes atransistor 201 (also referred to as a fourth transistor), a transistor202 (also referred to as a fifth transistor), a transistor 203 (alsoreferred to as a sixth transistor), and a transistor 204 (also referredto as a seventh transistor). However, this embodiment is not limited tothis example. Any of these transistors can be eliminated. Alternatively,any of these transistors can be replaced with any of a variety ofelements such as a capacitor, a resistor and a diode, or a circuit inwhich any of these elements are combined. Alternatively, a variety ofelements such as a transistor, a capacitor, a resistor and a diode or acircuit in which any of these elements are combined can be additionallyprovided.

Note that for example, the transistors 201 to 204 are n-channeltransistors. Specifically, in the case where the transistors 101 to 103in Embodiment 1 are n-channel transistors, it is preferable that thetransistors 201 to 204 be n-channel transistors. Accordingly, all thetransistors can be n-channel transistors. However, this embodiment isnot limited to this example. The transistors 201 to 204 can be p-channeltransistors.

Next, an example of a connection relation of the circuit 104 isdescribed. A first terminal of the transistor 201 is connected to thewiring 112 and a second terminal of the transistor 201 is connected tothe node B. A first terminal of the transistor 202 is connected to thewiring 116, a second terminal of the transistor 202 is connected to thenode B, and a gate of the transistor 202 is connected to the wiring 111.A first terminal of the transistor 203 is connected to the wiring 112, asecond terminal of the transistor 203 is connected to a gate of thetransistor 201, and a gate of the transistor 203 is connected to thewiring 112. A first terminal of the transistor 204 is connected to thewiring 116, a second terminal of the transistor 204 is connected to thegate of the transistor 201, and a gate of the transistor 204 isconnected to the wiring 111. However, this embodiment is not limited tothis example. A variety of connection structures can be employed.

Note that a connection portion of the gate of the transistor 201, thesecond terminal of the transistor 203, or the second terminal of thetransistor 204 is referred to as a node C. Note that the node C can bereferred to as a wiring or a terminal.

Note that as described in Embodiment 1, a variety of signals, voltages,or currents can be input to the wiring 111, the wiring 112, or thewiring 116. Here, for example, the signal OUT described in Embodiment 1is input to the wiring 111. For example, the signal IN1 described inEmbodiment 1 is input to the wiring 112. For example, the voltage V1described in Embodiment 1 is supplied to the wiring 116. However, thisembodiment is not limited to this example.

Next, examples of functions of the transistors 201 to 204 are described.The transistor 201 has a function of controlling timing when the signalIN2 is supplied to the node B in accordance with the potential of thenode C and can function as a bootstrap transistor or a switch. Thetransistor 202 has a function of controlling timing when the voltage V1is supplied to the node B by controlling the electrical conduction stateof the wiring 116 and the node B in accordance with the potential (thesignal OUT) of the wiring 111 and can function as a switch. Thetransistor 203 has a function of making the node C go into a floatingstate after increasing the potential of the node C and can function as adiode. The transistor 204 has a function of controlling timing when thevoltage V1 is supplied to the node C by controlling the electricalconduction state of the wiring 116 and the node C in accordance with thepotential (the signal OUT) of the wiring 111 and can function as aswitch. However, this embodiment is not limited to this example. Thetransistors 201 to 204 can have a variety of functions other than above.Alternatively, it is acceptable that these elements or circuits do nothave the above-described functions.

Next, operation of the circuit 104 is described with reference to FIG.1B and FIGS. 9B to 9F. FIG. 9B is an example of a schematic view of theoperation of the circuit 104 in the period T1. FIG. 9C is an example ofa schematic view of the operation of the circuit 104 in the period T2.FIG. 9D is an example of a schematic view of the operation of thecircuit 104 in the period T3. FIG. 9E is an example of a schematic viewof the operation of the circuit 104 in the period T4. FIG. 9F is anexample of a schematic view of the operation of the circuit 104 in theperiod T5.

First, for convenience, the operation is described in order from theoperation in the period T2. In the period T2, the signal IN2 goes intoan H level and the signal OUT goes into an H level. Since the signal OUTgoes into the H level, the transistor 202 and the transistor 204 areturned on. Then, the wiring 116 and the node B are brought intoelectrical conduction and the wiring 116 and the node C are brought intoelectrical conduction. Accordingly, since the voltage V1 is supplied tothe node B from the wiring 116 through the transistor 202, the potentialof the node B is decreased to V1. Then, since the voltage V1 is suppliedto the node C from the wiring 116 through the transistor 204, thepotential of the node C is decreased. The potential of the node C atthat time is determined by operating points of the transistor 203 andthe transistor 204. Here, for example, the potential of the node C has avalue smaller than the sum of the voltage V1 and the threshold voltageof the transistor 201 (Vth201), (V1+Vth201). Therefore, the transistor201 is turned off, whereby the wiring 112 and the node B are brought outof electrical conduction.

Next, in the period T3, the signal IN1 goes into an L level and thesignal OUT goes into an L level. Since the signal OUT goes into the Llevel, the transistor 202 and the transistor 203 are turned off.Therefore, the wiring 116 and the node B are brought out of electricalconduction and the wiring 116 and the node C are brought out ofelectrical conduction. Then, since the signal IN1 goes into the L level,the transistor 203 is turned off. Accordingly, the node C goes into afloating state, whereby the potential in the period T2 is maintained.Therefore, the transistor 201, is kept off.

Next, in the period T4, the signal IN1 goes into the H level and thesignal OUT is kept at the L level. Since the signal OUT is kept at the Llevel, the transistor 202 and the transistor 203 are kept off.Therefore, the wiring 116 and the node B are kept out of electricalconduction and the wiring 116 and the node C are kept out of electricalconduction. At that time, the signal IN1 is in the H level. Accordingly,the transistor 203 is turned on, whereby the wiring 112 and the node Care brought into electrical conduction. Therefore, since the signal IN1in the H level is supplied to the node C from the wiring 112 through thetransistor 203, the potential of the node C starts to increase. Afterthat, when the potential of the node C becomes equal to (V1+Vth201), thetransistor 201 is turned on. Then, the wiring 112 and the node B arebrought into electrical conduction. Therefore, since the signal IN1 inthe H level is supplied to the node B from the wiring 112 through thetransistor 201, the potential of the node B starts to increase. Afterthat, when the potential of the node C becomes equal to the valueobtained by subtracting the threshold voltage of the transistor 203(Vth203) from the potential of the signal IN1 in the H level (V2), thetransistor 203 is turned off. Therefore, the wiring 112 and the node Care brought out of electrical conduction. Then, the node C goes into afloating state, whereby the potential of the node C continues toincrease by capacitive coupling of the parasitic capacitance between thegate and the second terminal of the transistor 201, that is, bootstrapoperation. Then, if the potential of the node C gets higher than(V2+Vth201), the potential of the node B increases up to V2.

Next, in the period T5 or the period T1, the signal IN1 goes into the Llevel and the signal OUT is kept at the L level. Since the signal OUT iskept at the L level, the transistor 202 and the transistor 203 are keptoff. Therefore, the wiring 116 and the node B are kept out of electricalconduction and the wiring 116 and the node C are kept out of electricalconduction. Then, the signal IN1 goes into the L level. Accordingly, thetransistor 203 is turned off, whereby the wiring 112 and the node C arekept out of electrical conduction. Therefore, since the potential of thenode C goes into a floating state, the node C maintains potential whichexceeds (V2+Vth201). As a result, the transistor 201 is kept on, wherebythe wiring 112 and the node B are kept in electrical conduction.Therefore, since the signal IN1 in the L level is supplied to the node Bfrom the wiring 112 through the transistor 201, the potential of thenode B is decreased to V1. At that time, the node C is in the floatingstate, whereby the potential of the node C is decreased by capacitivecoupling of the parasitic capacitance between the gate and the secondterminal of the transistor 201 in many cases. Further, the potential ofthe node C is decreased by a value of an increase in the potential dueto the bootstrap operation in the period T4 in many cases.

The above is the description of the circuit 104 in FIG. 9A. The circuit104 in FIG. 9A can increase the potential of the node B to V2 by thebootstrap operation. Therefore, Vgs of the transistor 102 and thetransistor 103 in Embodiment 1 can be increased. As a result, since thechannel width of the transistor 102 and the transistor 103 can bereduced, reduction in a layout area can be achieved. Alternatively, evenif the threshold voltages of the transistor 102 and the transistor 103are increased, the transistors can be easily turned on. Alternatively,since the on-resistances of the transistor 102 and the transistor 103are reduced, the potential of the node A and the potential of the wiring111 can be easily maintained as V1.

Alternatively, all the transistors in the circuit 104 in FIG. 9A can ben-channel transistors or all the transistors in the circuit 104 in FIG.9A can be p-channel transistors. Accordingly, reduction in the number ofsteps, improvement in yield, improvement in reliability, or reduction incost can be achieved. Specifically, if all the transistors are n-channeltransistors, non-single-crystal semiconductors, microcrystallinesemiconductors, organic semiconductors, or oxide semiconductors can beused for semiconductor layers of the transistors. Accordingly, reductionin the number of steps, improvement in yield, improvement inreliability, or reduction in cost can be achieved. However, thisembodiment is not limited to this example. The circuit 104 in FIG. 9Acan include a CMOS circuit which includes a p-channel transistor and ann-channel transistor. Moreover, single crystal semiconductors orpolycrystalline semiconductors can be used for the semiconductor layersof the transistors.

Alternatively, in the circuit 104 in FIG. 9A, the transistors 202 to 204are turned off in at least one of the period T4 and the period T5.Therefore, since the transistor stays in an off state in one operationperiod, deterioration of characteristics of the transistor such asincrease in threshold voltage or decrease in mobility can be suppressed.Alternatively, an increase and a decrease in the potential of the node Care repeated in the period T4 and the period T5. Accordingly, sincepulses are input to the transistor 201, the deterioration ofcharacteristics of the transistor such as increase in threshold voltageor decrease in mobility can be suppressed. Specifically, if anon-single-crystal semiconductor, a microcrystalline semiconductor, anorganic semiconductor, an oxide semiconductor, or the like is used for asemiconductor layer of the transistor, the characteristics of thetransistor obviously deteriorates in many cases. However, in thesemiconductor device in FIG. 9A, a non-single-crystal semiconductor, amicrocrystalline semiconductor, an organic semiconductor, an oxidesemiconductor, or the like can be easily used for the semiconductorlayer of the transistor because deterioration of characteristics of thetransistor can be suppressed. However, this embodiment is not limited tothis example. A polycrystalline semiconductor or a single crystalsemiconductor can be used for the semiconductor layer.

Note that for example, the channel width of the transistor 203 ispreferably smaller than that of the transistor 204. This is because thepotential of the node C is made low when the transistor 203 and thetransistor 204 are turned on in the period T2. From a similar reason,for example, the channel length of the transistor 203 is preferablysmaller than that of the transistor 204. However, this embodiment is notlimited to this example. The channel width of the transistor 203 can belarger than that of the transistor 204. Alternatively, the channel widthof the transistor 203 can be smaller than that of the transistor 204.

Note that for example, the channel width of the transistor 204 ispreferably smaller than that of the transistor 202. This is because theload of the node B is heavier than that of the node C in many cases.From a similar reason, for example, the channel width of the transistor203 is preferably shorter than that of the transistor 201. However, thisembodiment is not limited to this example. The channel width of thetransistor 204 can be larger than that of the transistor 202.Alternatively, the channel width of the transistor 203 can be largerthan that of the transistor 201.

Note that for example, it is preferable that the channel width of thetransistor 201 be approximately equal to that of the transistor 202.This is because both of the transistor 201 and the transistor 202control the potential of the node C and have the same polarity. However,this embodiment is not limited to this example. The channel width of thetransistor 201 can be larger or smaller than that of the transistor 202.

Note that for example, the channel width of the transistor 201, thetransistor 202, the transistor 203, or the transistor 204 is preferablysmaller than that of the transistor 101, the transistor 102, or thetransistor 103 in Embodiment 1. However, this embodiment is not limitedto this example. The channel width of any of the transistors 201 to 204can be larger than that of the transistor 101, the transistor 102, orthe transistor 103 in FIG. 1A.

Note that for example, like in the case of the transistor 101 inEmbodiment 1, the parasitic capacitance between the gate and the secondterminal of the transistor 201 is preferably higher than the parasiticcapacitance between the gate and the first terminal of the transistor201. This is because the potential of the node C is easily increased bythe bootstrap operation in the period T4. Therefore, an area where aconductive layer which functions as a gate and a conductive layer whichfunctions as a source or drain on the second terminal side is preferablylarger than that on the first terminal side. However, this embodiment isnot limited to this example.

Note that a signal whose potential in an L level is lower than V1 can beinput to the terminal 104 b. In this manner, since backward bias can beapplied to the transistor 202 and the transistor 204, deterioration ofcharacteristics of the transistor 202 and the transistor 204 can beeased. Alternatively, a signal whose potential in an H level is lowerthan V2 can be input to the terminal 104 b. In this manner, since Vgscan be decreased when the transistor 202 and the transistor 204 are on,the deterioration of the characteristics of the transistor 202 and thetransistor 204 can be suppressed. In such a case, a signal whosepotential in an L level is lower than V1, a signal whose potential in anH level is lower than V2, or a signal whose potential in an L level islower than V1 and whose potential in an H level is lower than V2 can beinput to the wiring 111. However, this embodiment is not limited to thisexample. The terminal 104 b can be connected to a different wiring fromthe wiring 111 and the signal whose potential in the L level is lowerthan V1, the signal whose potential in the H level is lower than V2, orthe signal whose potential in the L level is lower than V1 and whosepotential in the H level is lower than V2 can be input to the differentwiring.

Note that like in Embodiment 1, a wiring can be divided into a pluralityof wirings. In addition, the same signal or voltage or different signalsor voltages can be input to the plurality of wirings. Alternatively, theplurality of wirings can be connected to the same wiring or element ordifferent wirings or elements. One example in FIG. 10A illustrates astructure in the case where the wiring 111 is divided into a pluralityof wirings of wirings 111A and 111B, the wiring 112 is divided into aplurality of wirings of wirings 112C and 112D, and the wiring 116 isdivided into a plurality of wirings of wirings 116E and 116F. Inaddition, the gate of the transistor 204 is connected to the wiring 111Aand the gate of the transistor 202 is connected to the wiring 111B. Thefirst terminal of the transistor 201 is connected to the wiring 112C andthe first terminal and the gate of the transistor 203 are connected tothe wiring 112D. The first terminal of the transistor 202 is connectedto the wiring 116E and the first terminal of the transistor 204 isconnected to the wiring 116F. However, this embodiment is not limited tothis example. Any one or two of the wiring 111, the wiring 112, and thewiring 116 can be divided into a plurality of wirings. Alternatively,different signals or voltages can be input to the gate and the firstterminal of the transistor 203. In that case, the gate and the firstterminal of the transistor 203 can be connected to different wirings.

Note that in FIG. 10A, the wirings 111A and 111B correspond to thewiring 111 in FIG. 9A. Therefore, like in the case of the wiring 111,the signal OUT can be input to the wirings 111A and 111B and the wirings111A and 111B can function as signal lines. However, this embodiment isnot limited to this example. Voltages such as the voltage V1 and thevoltage V2 can be supplied to the wirings 111A and 111B and the wirings111A and 111B can function as power supply lines. Alternatively,different signals or voltages can be input to the wirings 111A and 111B.Alternatively, a variety of signals, voltages, or currents other thanabove can be input to the wirings 111A and 111B.

Note that in FIG. 10A, the wirings 112C and 112D correspond to thewiring 112 in FIG. 9A. Therefore, like in the case of the wiring 112,the signal IN1 can be input to the wirings 112C and 112D and the wirings112C and 112D can function as signal lines. However, this embodiment isnot limited to this example. Voltages such as the voltage V1 and thevoltage V2 can be supplied to the wirings 112C and 112D and the wirings112C and 112D can function as power supply lines. Alternatively,different signals or voltages can be input to the wirings 112C and 112D.Alternatively, a variety of signals, voltages, or currents other thanabove can be input to the wirings 112C and 112D.

Note that in FIG. 10A, the wirings 116E and 116F correspond to thewiring 116 in FIG. 9A. Therefore, like in the case of the wiring 116,the voltage V1 can be supplied to the wirings 116E and 116F and thewirings 116E and 116F can function as power supply lines. However, thisembodiment is not limited to this example. A signal such as the signalOUT, the signal IN1, the signal IN2, the signal IN3, or signal IN4 canbe input to the wirings 116E and 116F so that the wirings 116E and 116Fcan function as signal lines. Alternatively, different signals orvoltages can be supplied to the wirings 116E and 116F. Alternatively, avariety of signals, voltages, or currents other than above can be inputto the wirings 116E and 116F.

Note that in FIG. 10A, a signal in an L level in the period T2 can beinput to the wirings 116E and 116F. For example, the signal IN2 can beinput to the wirings 116E and 116F. In that case, the wirings 116E and116F can be connected to the wiring 113 described in Embodiment 1. Inthis manner, since backward bias is applied to the transistor 202 andthe transistor 204, deterioration of characteristics of the transistor202 and the transistor 204 can be eased. However, this embodiment is notlimited to this example. The signal IN2 can be input to only one of thewirings 116E and 116F. In that case, only one of the wirings 116E and116F can be connected to the wiring 113. Alternatively, the signal IN3or the signal IN4 can be input to the wiring 116E and/or the wiring116F. In that case, the wiring 116E and/or the wiring 116F can beconnected to the wiring 114 or the wiring 115 described in Embodiment 1.

Note that in FIG. 10A, the signal IN2 can be input to the wiring 112D.In that case, the gate and the first terminal of the transistor 203 canbe connected to the wiring 113. In this manner, in the period T3, thepotential of the node C becomes equal to a value obtained by subtractingthe threshold voltage of the transistor 203 (Vth203) from the potentialof the signal IN2 in an H level (V2), (V2 Vth203). After that, in theperiod T4, since the signal IN1 goes into an H level, the potential ofthe node C is further increased from (V2 Vth203) by bootstrap operation.Accordingly, since the potential of the node C is increased, Vgs of thetransistor 201 can be increased. As a result, a falling time and arising time of an output signal (the potential of the node B) from thecircuit 104 can be shortened. Alternatively, delay of the output signalfrom the circuit 104 can be suppressed. However, this embodiment is notlimited to this example. The voltage V2 can be supplied to the wiring112D.

Note that as shown in FIG. 10B, a capacitor 221 can be additionallyconnected between the gate and the second terminal of the transistor201. In this manner, like in FIG. 6A, the potential of the node C can beincreased. However, this embodiment is not limited to this example. Likein FIG. 6A, for the capacitor 221, a transistor whose first terminal andsecond terminal are connected to the node B and whose gate is connectedto the node C can be used as a MOS capacitor.

Note that like in FIG. 10B, the capacitor 221 can be additionallyconnected between the gate and the second terminal of the transistor 201in FIG. 10A. Alternatively, a transistor whose first terminal and secondterminal are connected to the node B and whose gate is connected to thenode C can be additionally provided.

Note that as shown in FIG. 10C, the transistor 204 can be eliminated.Alternatively, as shown in FIG. 10D, the transistor 202 can beeliminated. In this manner, the number of transistors can be reduced.Therefore, reduction in a layout area, improvement in yield, or the likecan be achieved. However, this embodiment is not limited to thisexample. Both of the transistor 202 and the transistor 204 can beeliminated.

Note that like in FIG. 10C or FIG. 10D, the transistor 202 and/or thetransistor 204 can be eliminated in FIGS. 10A and 10B.

Note that as shown in FIG. 10E, the transistor 202 can be replaced witha diode 202 a one terminal (also referred to as a positive electrode) ofwhich is connected to the node B and the other terminal (also referredto as a negative electrode) of which is connected to the wiring 111.Alternatively, the transistor 203 can be replaced with a diode 203 a oneterminal (also referred to as a positive electrode) of which isconnected to the node C and the other terminal (also referred to as anegative electrode) of which is connected to the wiring 111. In thatcase, an inverted signal of the signal OUT or an inverted signal of thepotential of the node A can be input to the terminal 104 b of thecircuit 104. In order to achieve this, the wiring 111 or the node A canbe connected to the terminal 104 b of the circuit 104 through a circuitwhich has a function of inverting and outputting an input signal, suchas an inverter circuit, a NAND circuit, or a NOR circuit. However, thisembodiment is not limited to this example. One of the transistor 202 andthe transistor 204 can be replaced with a diode. Alternatively, thediode 202 a and/or the diode 203 a can be additionally provided.

Note that like in FIG. 10E, the transistor 202 can be replaced with thediode 202 a one terminal of which is connected to the node B and theother terminal of which is connected to the wiring 111 in FIGS. 10A to10D. Alternatively, the transistor 203 can be replaced with the diode203 a one terminal of which is connected to the node C and the otherterminal of which is connected to the wiring 111. Alternatively, thediode 202 a and/or the diode 203 a can be additionally provided.

Note that although not shown, the first terminal of the transistor 202is connected to the wiring 111, the second terminal of the transistor202 is connected to the node B, and the gate of the transistor 202 isconnected to the wiring 111 or the node B, whereby the transistor 202can be diode-connected. Alternatively, the first terminal of thetransistor 204 is connected to the wiring 111, the second terminal ofthe transistor 204 is connected to the node C, and the gate of thetransistor 204 is connected to the wiring 111 or the node C, whereby thetransistor 204 can be diode-connected. However, this embodiment is notlimited to this example. One of the transistor 202 and the transistor204 can be diode-connected.

Note that as shown in FIG. 10F, p-channel transistors can be used as thetransistors 201 to 204. Specifically in the case where p-channeltransistors are used as the transistors 101 to 103 in FIG. 1A, p-channeltransistors are preferably used as the transistors 201 to 204. Atransistor 201 p, a transistor 202 p, a transistor 203 p, and atransistor 204 p correspond to the transistor 201, the transistor 202,the transistor 203, and the transistor 204, respectively, and arep-channel transistors.

Note that like in FIG. 10F, p-channel transistors can be used as thetransistors 201 to 204 in FIGS. 10A to 10E.

Note that as described above, the structure of the circuit 104 in thisembodiment can be applied to the circuit 104 included in the circuit 100in Embodiment 1. FIG. 11 illustrates a structure in the case where oneexample of the circuit 104 in FIG. 9A is applied to the circuit 104included in the circuit 100 in FIG. 7C, for example. However, thisembodiment is not limited to this example. The circuit 104 in the caseof FIG. 9A, FIG. 10A, FIG. 10B, FIG. 10C, FIG. 10D, FIG. 10E, or FIG.10F, or a combination thereof can be used as the circuit 104 included inthe circuit 100 in the case of FIG. 1A, FIG. 5A, FIG. 5B, FIG. 6A, FIG.6B, FIG. 6C, FIG. 7A, FIG. 7B, or FIG. 8A, or a combination thereof.

Embodiment 3

In this embodiment, a specific example of the circuit 105 is described.Note that the circuit 105 can be referred to as a semiconductor device,a driver circuit, or a gate driver. Note that description of the contentdescribed in Embodiment 1 or 2 is omitted. Note that the contentdescribed in Embodiment 1 or 2 can be freely combined with a contentdescribed in this embodiment.

First, one example of the circuit 105 is described with reference toFIG. 12A. In the example in FIG. 9A, the circuit 105 includes atransistor 301 (also referred to as an eighth transistor), a transistor302 (also referred to as a ninth transistor), a transistor 303 (alsoreferred to as a tenth transistor), a transistor 304 (also referred toas an eleventh transistor), and a transistor 305 (also referred to as atwelfth transistor). However, this embodiment is not limited to thisexample. Any of these transistors can be eliminated. Alternatively, anyof these transistors can be replaced with any of a variety of elementssuch as a capacitor, a resistor and a diode, or a circuit in which theseelements are combined. Alternatively, a variety of elements such as atransistor, a capacitor, a resistor, and a diode or a circuit in whichthese elements are combined can be additionally provided.

Note that for example, the transistors 301 to 305 are n-channeltransistors. Specifically, in the case where the transistors 101 to 103in Embodiment 1 and the transistors 201 to 204 in Embodiment 2 aren-channel transistors, it is preferable that the transistors 301 to 305be n-channel transistors. Accordingly, all the transistors can have thesame polarity. However, this embodiment is not limited to this example.The transistors 301 to 305 can be p-channel transistors.

Next, an example of a connection relation of the circuit 105 in FIG. 12Ais described. A first terminal of the transistor 301 is connected to thewiring 114, a second terminal of the transistor 301 is connected to thenode A, and a gate of the transistor 301 is connected to the wiring 114.A first terminal of the transistor 302 is connected to the wiring 114, asecond terminal of the transistor 302 is connected to the node A, and agate of the transistor 302 is connected to the wiring 113. A firstterminal of the transistor 303 is connected to the wiring 116, a secondterminal of the transistor 303 is connected to the node A, and a gate ofthe transistor 303 is connected to the wiring 115. A first terminal ofthe transistor 304 is connected to the wiring 116, a second terminal ofthe transistor 304 is connected to the wiring 111, and a gate of thetransistor 304 is connected to the wiring 115. A first terminal of thetransistor 305 is connected to the wiring 116, a second terminal of thetransistor 305 is connected to the wiring 111, and a gate of thetransistor 305 is connected to the wiring 113. However, this embodimentis not limited to this example. A variety of connection structures otherthan above can be employed.

Note that as described in Embodiment 1, a variety of signals, voltages,or currents can be input to the wiring 113, the wiring 114, the wiring115, or the wiring 116. Here, for example, the signal 11\12 described inEmbodiment 1 is input to the wiring 113; the signal IN3 described inEmbodiment 1 is input to the wiring 114; the signal IN4 shown in FIG. 1Bor FIG. 3A is input to the wiring 115; and the voltage V1 is supplied tothe wiring 116. However, this embodiment is not limited to this example.

Next, examples of functions of the transistors 301 to 305 are described.The transistor 301 controls timing when the signal IN2 in an H level issupplied to the node A in accordance with the signal IN3 and canfunction as a diode. Alternatively, the transistor 301 has a function ofcontrolling timing when the signal IN3 is supplied to the node A bycontrolling the electrical conduction state of the wiring 114 and thenode A in accordance with the potential of the node A. The transistor302 has a function of controlling timing when the signal IN3 is suppliedto the node A by controlling the electrical conduction state of thewiring 114 and the node A in accordance with the signal IN2 and canfunction as a switch. The transistor 303 has a function of supplying thevoltage V1 to the node A by controlling the electrical conduction stateof the wiring 116 and the node A in accordance with the signal IN4 andcan function as a switch. The transistor 304 has a function of supplyingthe voltage V1 to the wiring 111 by controlling the electricalconduction state of the wiring 116 and the wiring 111 in accordance withthe signal IN4 and can function as a switch. The transistor 305 has afunction of supplying the voltage V1 to the wiring 111 by controllingthe electrical conduction state of the wiring 116 and the wiring 111 inaccordance with the signal IN2 and can function as a switch. However,this embodiment is not limited to this example. The transistors 301 to305 can have a variety of functions. Alternatively, it is acceptablethat these elements or circuits do not have the above-describedfunctions.

Next, operation of the circuit 105 is described with reference to FIG.1B, FIGS. 12B and 12C, and FIGS. 13A to 13C. FIG. 12B is an example of aschematic view of the operation of the circuit 105 in the period T1.FIG. 12C is an example of a schematic view of the operation of thecircuit 105 in the period T2. FIG. 13A is an example of a schematic viewof the operation of the circuit 105 in the period T3. FIG. 13B is anexample of a schematic view of the operation of the circuit 105 in theperiod T4. FIG. 13C is an example of a schematic view of the operationof the circuit 105 in the period T5.

First, in the period T1, the signal IN2 goes into an H level, the signalIN3 goes into an H level, and the signal IN4 goes into an L level. Sincethe signal IN3 is in the H level, the transistor 301 is turned on. Atthe same time, since the signal IN2 is in the H level, the transistor302 and the transistor 305 are turned on. Then, the wiring 114 and thenode A are brought into electrical conduction, whereby the signal IN3 issupplied to the node A from the wiring 114 through the transistor 301and the transistor 302. Therefore, the potential of the node A starts toincrease. Similarly, since the wiring 116 and the wiring 111 are broughtinto electrical conduction, the voltage V1 is supplied to the wiring 111from the wiring 116 through the transistor 305. Therefore, the potentialof the wiring 111 becomes V1. At that time, since the signal IN4 is inthe L level, the transistor 303 and the transistor 304 are turned off.Therefore, the wiring 116 and the node A are brought out of electricalconduction, whereby the wiring 116 and the wiring 111 are brought out ofelectrical conduction. After that, when the potential of the node Abecomes equal to a value obtained by subtracting the threshold voltageof the transistor 301 (Vth301) from the potential of the wiring 114(V2), (V2−Vth301), the transistor 301 is turned off. Similarly, when thepotential of the node A becomes equal to a value obtained by subtractingthe threshold voltage of the transistor 302 (Vth302) from the potentialof the wiring 113 (V2), (V2−Vth302), the transistor 302 is turned off.Therefore, the wiring 114 and the node A are brought out of electricalconduction. Here, for example, when the potential of the node A becomesequal to (V2−Vth301), the transistor 301 and the transistor 302 areturned off. Therefore, the node A goes into a floating state while thepotential is maintained as (V2−Vth301).

Next, in the period T2, the signal IN3 goes into an L level, the signalIN4 goes into an L level, and the signal IN5 is kept at the L level.Since the signal IN3 is in the L level, the transistor 301 is kept off.At the same time, since the signal IN2 is in the L level, the transistor302 is kept off and the transistor 303 is turned off. Therefore, thewiring 114 and the node A are kept out of electrical conduction and thewiring 116 and the wiring 111 are brought out of electrical conduction.At that time, since the signal IN4 is kept at the L level, thetransistor 303 and the transistor 304 are kept off. Accordingly, thewiring 116 and the node A are kept out of electrical conduction and thewiring 116 and the wiring 111 are kept out of electrical conduction.

Next, in the period T3, the signal IN2 goes into an H level, the signalIN3 is kept at the L level, and the signal IN4 goes into an H level.Since the signal IN3 is kept at the L level, the transistor 301 is keptoff. Then, since the signal IN2 is in the H level, the transistor 302and the transistor 304 are turned on. Then, the wiring 114 and the nodeA are brought into electrical conduction, whereby the signal IN3 in theL level is supplied to the node A from the wiring 114 through thetransistor 302. Similarly, since the wiring 116 and the wiring 111 arebrought into electrical conduction, the voltage V1 is supplied to thewiring 111 from the wiring 116 through the transistor 305. At that time,since the signal IN4 is in the H level, the transistor 303 and thetransistor 304 are turned on. Then, the wiring 116 and the node A arebrought into electrical conduction, whereby the voltage V1 is suppliedto the node A from the wiring 116 through the transistor 303. Similarly,since the wiring 116 and the wiring 111 are brought into electricalconduction, the voltage V1 is supplied to the wiring 111 from the wiring116 through the transistor 304. Therefore, the potential of the node Adecreases to V1 and the potential of the wiring 111 decreases to V1.

Next, in the period T4, the signal IN2 goes into the L level, the signalIN3 is kept at the L level, and the signal IN4 goes into the L level.Since the signal IN3 is kept at the L level, the transistor 301 is keptoff. In addition, since the signal IN2 is in the L level, the transistor302 and the transistor 305 are turned off. Therefore, the wiring 114 andthe node A are kept out of electrical conduction and the wiring 116 andthe wiring 111 are brought out of electrical conduction. At the sametime, since the signal IN4 is in the L level, the transistor 303 and thetransistor 304 are turned off. Accordingly, the wiring 116 and the nodeA are kept out of electrical conduction and the wiring 116 and thewiring 111 are kept out of electrical conduction. In this manner, in theperiod T4, a signal, voltage, or the like is not supplied to the node Aor the wiring 111 from the circuit 105 in many cases.

Next, in the period T5, the signal IN2 goes into the H level, the signalIN3 is kept at the L level, and the signal IN4 is kept at the L level.Since the signal IN4 is kept at the L level, the transistor 303 and thetransistor 304 are tuned off. Therefore, the wiring 116 and the node Aare kept out of electrical conduction and the wiring 116 and the wiring111 are kept out of electrical conduction. Similarly, since the signalIN3 is kept at the L level, the transistor 301 is kept off. At thattime, since the signal IN2 is in the H level, the transistor 302 and thetransistor 305 are turned on. Then, the wiring 114 and the node A arebrought into electrical conduction, whereby the signal IN3 in the Llevel is supplied to the node A from the wiring 114 through thetransistor 302. Therefore, the potential of the node A is maintained asV1. Similarly, since the wiring 116 and the wiring 111 are brought intoelectrical conduction, the voltage V1 is supplied to the wiring 111 fromthe wiring 116 through the transistor 305. Therefore, the potential ofthe wiring 111 is maintained as V1.

The above is the description of the circuit 105 in FIG. 12A. In FIG.12A, all the transistors in the circuit 105 can be n-channel transistorsor all the transistors in the circuit 105 can be p-channel transistors.Accordingly, reduction in the number of steps, improvement in yield,improvement in reliability, or reduction in cost can be achieved.Specifically, if all the transistors are n-channel transistors,non-single-crystal semiconductors, microcrystalline semiconductors,organic semiconductors, oxide semiconductors, or the like can be usedfor semiconductor layers of the transistors. Accordingly, reduction inthe number of steps, improvement in yield, improvement in reliability,or reduction in cost can be achieved. However, this embodiment is notlimited to this example. The circuit 105 in FIG. 12A can include a CMOScircuit which includes a p-channel transistor and an n-channeltransistor. Alternatively, single crystal semiconductors orpolycrystalline semiconductors can be used for the semiconductor layersof the transistors.

Alternatively, in the circuit 105 in FIG. 12A, the transistors 301 to305 are turned off in at least one of the period T4 and the period T5.Therefore, since the transistor stays in an off state in one operationperiod, deterioration of characteristics of the transistor, such asincrease in threshold voltage or reduction in mobility, can besuppressed. Specifically, if a non-single-crystal semiconductor, amicrocrystalline semiconductor, an organic semiconductor, an oxidesemiconductor, or the like is used for a semiconductor layer of thetransistor, the characteristics of the transistor obviously deterioratesin many cases. However, in the circuit 105 in FIG. 12A, anon-single-crystal semiconductor, a microcrystalline semiconductor, anorganic semiconductor, an oxide semiconductor, or the like can be easilyused for the semiconductor layer of the transistor because deteriorationof characteristics of the transistor can be suppressed. However, thisembodiment is not limited to this example. A polycrystallinesemiconductor or a single crystal semiconductor can be used for thesemiconductor layer.

Note that for example, the channel width of the transistor 305 ispreferably larger than that of the transistor 302. Alternatively, thechannel width of the transistor 304 is preferably larger than that ofthe transistor 303, for example. This is because, since the load of thewiring 111 is heavier than that of the node A in many cases, the drivingcapability of a transistor which supplies a signal, voltage, or the liketo the wiring 111 is higher than that of a transistor which supplies asignal, voltage, or the like to the node A in many cases. In addition,this is because the transistor 305 and the transistor 304 each have afunction of supplying a signal or voltage to the wiring 111, and thetransistor 302 and the transistor 303 each have a function of supplyinga signal or voltage to the node A. However, this embodiment is notlimited to this example. The channel width of the transistor 305 can besmaller than that of the transistor 302. Alternatively, for example, thechannel width of the transistor 304 can be smaller than that of thetransistor 303. This is because noise is easily generated in the node Aby the parasitic capacitance between the first terminal and the gate ofthe transistor 101 in Embodiment 1; then, the transistor 101 is turnedon by the noise and the potential of the wiring 111 is increased in somecases.

Note that for example, the channel width of the transistor 303 ispreferably larger than that of the transistor 302. Alternatively, thechannel width of the transistor 304 is preferably larger than that ofthe transistor 305, for example. In this manner, adverse effect of noisegenerated in the node A and the wiring 111 can be suppressed. However,this embodiment is not limited to this example. The channel width of thetransistor 303 can be smaller than that of the transistor 302.Alternatively, the channel width of the transistor 304 can be smallerthan that of the transistor 305.

Note that a signal whose potential in an L level is lower than V1 can beinput to the terminals 105 a and 105 c. In this manner, since backwardbias can be applied to the transistors 302 to 305, deterioration ofcharacteristics of the transistors 302 to 305 can be eased.Alternatively, a signal whose potential in an H level is lower than V2can be input to the terminals 105 a and 105 c. In this manner, since Vgscan be decreased when the transistors 302 to 305 are on, thedeterioration of the characteristics of the transistors 302 to 305 canbe suppressed. In such a case, a signal whose potential in an L level islower than V1, a signal whose potential in an H level is lower than V2,or a signal whose potential in an L level is lower than V1 and whosepotential in an H level is lower than V2 can be input to the wirings 113and 115. However, this embodiment is not limited to this example. Thesignal whose potential in the L level is lower than V1, the signal whosepotential in the H level is lower than V2, or the signal whose potentialin the L level is lower than V1 and whose potential in the H level islower than V2 can be input to one of the terminals 105 a and 105 c. Inthat case, the signal whose potential in the L level is lower than V1,the signal whose potential in the H level is lower than V2, or thesignal whose potential in the L level is lower than V1 and whosepotential in the H level is lower than V2 can be input to one of thewirings 113 and 115. Alternatively, the terminal 105 a can be connectedto a different wiring from the wiring 113 and the signal whose potentialin the L level is lower than V1, the signal whose potential in the Hlevel is lower than V2, or the signal whose potential in the L level islower than V1 and whose potential in the H level is lower than V2 can beinput to the wiring. Alternatively, the terminal 105 c can be connectedto a different wiring from the wiring 115 and the signal whose potentialin the L level is lower than V1, the signal whose potential in the Hlevel is lower than V2, or the signal whose potential in the L level islower than V1 and whose potential in the H level is lower than V2 can beinput to the wiring.

Note that a signal which goes into an L level in the period T1, theperiod T3, and the period T4 can be input to the terminal 105 d. Forexample, the signal IN2 can be input to the terminal 105 d. In thatcase, the terminal 105 d can be connected to the wiring 113.Accordingly, since backward bias can be applied to the transistor 303,the transistor 304, or the transistor 305, deterioration ofcharacteristics of the transistor 303, the transistor 304, or thetransistor 305 can be eased.

Note that like in Embodiment 1 or 2, the wiring can be divided into aplurality of wirings. In addition, the same signal or voltage ordifferent signals or voltages can be input to the plurality of wirings.Alternatively, the plurality of wirings can be connected to the samewiring or element or different wirings or elements. One example in FIG.14A illustrates a structure in the case where the wiring 113 is dividedinto a plurality of wirings of wirings 113A and 113B, the wiring 114 isdivided into a plurality of wirings of wirings 114A and 114B, the wiring115 is divided into a plurality of wirings of wirings 115A and 115B, andthe wiring 116 is divided into a plurality of wirings of wirings 116G to116I. In addition, the gate of the transistor 302 is connected to thewiring 113A and the gate of the transistor 305 is connected to thewiring 113B. The first terminal of the transistor 302 is connected tothe wiring 114A. The first terminal and the gate of the transistor 301are connected to the wiring 114B. The gate of the transistor 303 isconnected to the wiring 115A. The gate of the transistor 304 isconnected to the wiring 115B. The first terminal of the transistor 303is connected to the wiring 116G The first terminal of the transistor 304is connected to the wiring 116H. The first terminal of the transistor305 is connected to the wiring 116I. However, this embodiment is notlimited to this example. Any one, two, or three of the wiring 113, thewiring 114, the wiring 115, and the wiring 116 can be divided into aplurality of wirings.

Note that in FIG. 14A, the wirings 113A and 113B correspond to thewiring 113 in FIG. 12A. Therefore, like in the case of the wiring 113,the signal IN2 can be input to the wirings 113A and 113B and the wirings113A and 113B can function as signal lines. However, this embodiment isnot limited to this example. Voltage such as the voltage V1 or thevoltage V2 can be supplied to the wirings 113A and 113B so that thewirings 113A and 113B can function as power supply lines. Alternatively,different signals or voltages can be input to the wirings 113A and 113B.Alternatively, a variety of signals, voltages, or currents other thanabove can be input to the wirings 113A and 113B.

Note that in FIG. 14A, the wirings 114A and 114B correspond to thewiring 114 in FIG. 12A. Therefore, like in the case of the wiring 114,the signal IN3 can be input to the wirings 114A and 114B and the wirings114A and 114B can function as signal lines. However, this embodiment isnot limited to this example. Voltage such as the voltage V1 or thevoltage V2 can be supplied to the wirings 114A and 114B so that thewirings 114A and 114B can function as power supply lines. Alternatively,different signals or voltages can be input to the wirings 114A and 114B.Alternatively, a variety of signals, voltages, or currents other thanabove can be input to the wirings 114A and 114B.

Note that in FIG. 14A, the wirings 115A and 115B correspond to thewiring 115 in FIG. 12A. Therefore, like in the case of the wiring 115,the signal IN4 can be input to the wirings 115A and 115B and the wirings115A and 115B can function as signal lines. However, this embodiment isnot limited to this example. Voltage such as the voltage V1 or thevoltage V2 can be supplied to the wirings 115A and 115B so that thewirings 115A and 115B can function as power supply lines. Alternatively,different signals or voltages can be supplied to the wirings 115A and115B. Alternatively, a variety of signals, voltages, or currents can beinput to the wirings 115A and 115B.

Note that in FIG. 14A, the wirings 116G to 116I correspond to the wiring116 in FIG. 12A. Therefore, like in the case of the wiring 116, thevoltage V1 can be supplied to the wirings 116G to 116I and the wirings116G to 116I can function as power supply lines. However, thisembodiment is not limited to this example. A signal such as the signalOUT, the signal IN1, the signal IN2, the signal IN3, or the signal IN4can be input to the wirings 116G to 116I so that the wirings 116G to116I can function as signal lines. Alternatively, different signals orvoltages can be supplied to the wirings 116G to 116I. Alternatively, avariety of signals, voltages, or currents can be input to the wirings116G to 116I.

Note that in FIG. 14A, a signal in an L level in the period T3 can beinput to the wirings 116G and 116H. For example, the signal IN2 can beinput to the wirings 116G and 116H. In that case, the wirings 116G and116H can be connected to the wiring 112 described in Embodiments 1 and2. In this manner, since backward bias can be applied to the transistor303 and the transistor 304, deterioration of characteristics of thetransistor 303 and the transistor 304 can be suppressed. However, thisembodiment is not limited to this example. The signal IN2 can be inputto only one of the wirings 116G and 116H. Alternatively, the signal OUTor the signal IN3 can be input to the wiring 116G and/or the wiring116H. In that case, the wiring 116G and/or the wiring 116H can beconnected to the wiring 111 or the wiring 114 described in Embodiments 1and 2.

Note that in FIG. 14A, a signal in an L level in the period T1, theperiod T3, and the period T5 can be input to the wiring 116I. Forexample, the signal IN2 can be input to the wiring 116I. In that case,the wiring 116I can be connected to the wiring 112 described inEmbodiments 1 and 2. In this manner, since backward bias can be appliedto the transistor 305, deterioration of characteristics of thetransistor 305 can be suppressed. However, this embodiment is notlimited to this example.

Note that as shown in FIG. 14B, the transistor 303 and the transistor304 can be eliminated. In this manner, the number of transistors can bereduced. Therefore, reduction in a layout area, improvement in yield, orthe like can be achieved. However, this embodiment is not limited tothis example. Only one of the transistor 303 and the transistor 304 canbe eliminated.

Note that like in FIG. 14B, the transistor 303 and/or the transistor 304can be eliminated in FIG. 14A.

Note that as shown in FIG. 14C, the transistor 305 can be eliminated. Inthis manner, the number of transistors can be reduced. Therefore,reduction in a layout area, improvement in yield, or the like can beachieved. However, this embodiment is not limited to this example.

Note that like in FIG. 14C, the transistor 305 can be eliminated inFIGS. 14A and 14B.

Note that as shown in FIG. 15A, the transistor 302 can be eliminated. Inthis manner, the number of transistors can be reduced. Therefore,reduction in a layout area, improvement in yield, or the like can beachieved. However, this embodiment is not limited to this example.

Note that like in FIG. 15A, the transistor 302 can be eliminated inFIGS. 14A to 14C.

Note that as shown in FIG. 15B, the transistor 301 can be eliminated. Inthis manner, the number of transistors can be reduced. Therefore,reduction in a layout area, improvement in yield, or the like can beachieved. However, this embodiment is not limited to this example.

Note that like in FIG. 15B, the transistor 301 can be eliminated inFIGS. 14A to 14C and FIG. 15A.

Note that as shown in FIG. 16A, the transistor 303 can be replaced witha diode 303 a one terminal (also referred to as a positive electrode) ofwhich is connected to the node A and the other terminal (also referredto as a negative electrode) of which is connected to the wiring 115.Alternatively, the transistor 304 can be replaced with a diode 304 a oneterminal (also referred to as a positive electrode) of which isconnected to the wiring 111 and the other terminal (also referred to asa negative electrode) of which is connected to the wiring 115. However,this embodiment is not limited to this example. One of the transistor303 and the transistor 304 can be replaced with a diode. Alternatively,the diode 303 a and/or the diode 304 a can be additionally provided.

Note that like in FIG. 16A, the transistor 303 can be replaced with thediode 303 a one terminal of which is connected to the node A and theother terminal of which is connected to the wiring 115 in FIGS. 14A to14C and FIGS. 15A and 15B. Alternatively, the transistor 304 can bereplaced with the diode 304 a one terminal of which is connected to thewiring 111 and the other terminal of which is connected to the wiring115. Alternatively, the diode 303 a and/or the diode 304 a can beadditionally provided.

Note that although not shown, in FIGS. 14A to 14C, FIGS. 15A and 15B,and FIG. 16A, the first terminal of the transistor 303 is connected tothe wiring 115, the second terminal of the transistor 303 is connectedto the node A, and the gate of the transistor 303 is connected to thenode A, whereby the transistor 303 can be diode-connected.Alternatively, the first terminal of the transistor 304 is connected tothe wiring 115, the second terminal of the transistor 304 is connectedto the wiring 111, and the gate of the transistor 304 is connected tothe wiring 111, whereby the transistor 304 can be diode-connected.However, this embodiment is not limited to this example. One of thetransistor 303 and the transistor 304 can be diode-connected.

Note that as shown in FIG. 16B, the transistor 305 can be replaced witha diode 305 a one terminal (also referred to as a positive electrode) ofwhich is connected to the wiring 111 and the other terminal (alsoreferred to as a negative electrode) of which is connected to the wiring113. However, this embodiment is not limited to this example. The diode305 a can be additionally provided.

Note that like in FIG. 16B, the transistor 305 can be replaced with thediode 305 a one terminal (also referred to as a positive electrode) ofwhich is connected to the wiring 111 and the other terminal (alsoreferred to as a negative electrode) of which is connected to the wiring113 in FIGS. 14A to 14C, FIGS. 15A and 15B, and FIG. 16A. Alternatively,the diode 305 a can be additionally provided.

Note that although not shown, in FIGS. 14A to 14C, FIGS. 15A and 15B,and FIGS. 16A and 16B, the first terminal of the transistor 305 isconnected to the wiring 113, the second terminal of the transistor 305is connected to the wiring 111, and the gate of the transistor 305 isconnected to the wiring 111, whereby the transistor 305 can bediode-connected. However, this embodiment is not limited to thisexample.

Note that as shown in FIG. 17A, the gate of the transistor 301 can beconnected to a wiring 117. Therefore, the circuit 105 can additionallyinclude a terminal 105 g. In addition, the wiring 117 is connected tothe gate of the transistor 301 through the terminal 105 g. The voltageV2 is supplied to the wiring 117 and the wiring 117 can function as apower supply line. However, this embodiment is not limited to thisexample. The first terminal of the transistor 301 can be connected tothe wiring 117 and the gate of the transistor 301 can be connected tothe wiring 114. Alternatively, a signal which goes into an H level inthe period T2 can be input to the wiring 117 and the wiring 117 canfunction as a signal line. For example, the signal IN2 can be input tothe wiring 117 and the wiring 117 can be connected to the wiring 113.Alternatively, a variety of signals, voltages, or currents other thanabove can be input to the wiring 117.

Note that like in FIG. 17A, the gate of the transistor 301 or the firstterminal of the transistor 301 can be connected to the wiring 117 inFIGS. 14A to 14C, FIGS. 15A and 15B, and FIGS. 16A and 16B.

Note that as shown in FIG. 17B, a transistor 306 and a transistor 307can be additionally provided. For example, the transistor 306 and thetransistor 307 have the same polarity as the transistors 301 to 305 inmany cases and are n-channel transistors. A first terminal of thetransistor 306 is connected to the wiring 116, a second terminal of thetransistor 306 is connected to the node A, and a gate of the transistor306 is connected to a wiring 118. A first terminal of the transistor 307is connected to the wiring 116, a second terminal of the transistor 307is connected to the wiring 111, and a gate of the transistor 307 isconnected to the wiring 118. For example, the signal IN5 is input to thewiring 118 and the wiring 118 can function as a signal line. Thetransistor 306 has a function of controlling timing when the voltage V1is supplied to the node A by controlling the electrical conduction stateof the wiring 116 and the node A in accordance with the signal IN5 orthe potential of the wiring 115 and can function as a switch. Thetransistor 307 has a function of controlling timing when the voltage V1is supplied to the wiring 111 by controlling the electrical conductionstate of the wiring 116 and the wiring 111 in accordance with the signalIN5 or the potential of the wiring 115 and can function as a switch. Forexample, the signal IN5 functions as an all-stages-reset signal in manycases. However, this embodiment is not limited to this example. Only oneof the transistor 306 and the transistor 307 can be additionallyprovided.

Note that as shown in FIG. 17B, the transistor 306 and/or the transistor307 can be additionally provided in FIGS. 14A to 14C, FIGS. 15A and 15B,FIGS. 16A and 16B, and FIG. 17A. The first terminal of the transistor306 is connected to the wiring 116, the second terminal of thetransistor 306 is connected to the node A, and the gate of thetransistor 306 is connected to a wiring 118. The first terminal of thetransistor 307 is connected to the wiring 116, the second terminal ofthe transistor 307 is connected to the wiring 111, and the gate of thetransistor 307 is connected to the wiring 118.

Note that as shown in FIG. 17C, p-channel transistors can be used as thetransistors 301 to 305. Specifically in the case where p-channeltransistors are used as the transistors 101 to 103 in Embodiment 1 andthe transistors 201 to 204 in Embodiment 2, p-channel transistors arepreferably used as the transistors 301 to 305. A transistor 301 p, atransistor 302 p, a transistor 303 p, a transistor 304 p, and atransistor 305 p correspond to the transistor 301, the transistor 302,the transistor 303, the transistor 304, and the transistor 305,respectively.

Note that like in FIG. 17C, p-channel transistors can be used as thetransistors 301 to 305 in FIGS. 14A to 14C, FIGS. 15A and 15B, FIGS. 16Aand 16B, and FIGS. 17A and 17B.

Note that as described above, the structure of the circuit 105 in thisembodiment can be applied to the circuit 105 included in the circuit 100described in Embodiment 1. FIG. 18A illustrates a structure in the casewhere one example of the circuit 105 in FIG. 12A is applied to thecircuit 105 included in the circuit 100 in FIG. 1A, for example. Inaddition, FIG. 18B illustrates a structure in the case where one exampleof the circuit 105 in FIG. 12A is applied to the circuit 105 included inthe circuit 100 in FIG. 1A and one example of the circuit 104 in FIG. 9Ais applied to the circuit 104 included in the circuit 100 in FIG. 1A,for example. However, this embodiment is not limited to this example.The circuit 105 in the case of FIG. 12A, FIG. 14A, FIG. 14B, FIG. 14C,FIG. 15A, FIG. 15B, FIG. 16A, FIG. 16B, FIG. 17A, FIG. 17B, or FIG. 17C,or a combination thereof can be used as the circuit 105 included in thecircuit 100 in the case of FIG. 1A, FIG. 5A, FIG. 5B, FIG. 6A, FIG. 6B,FIG. 6C, FIG. 7A, FIG. 7B, or FIG. 8A, or a combination thereof.

Embodiment 4

In this embodiment, one example of a shift register is described. Theshift register in this embodiment can include the semiconductor devicein Embodiments 1 to 3. Note that the shift register can be referred toas a semiconductor device or a gate driver. Note that description of thecontent described in Embodiments 1 to 3 is omitted. Note that thecontent described in Embodiments 1 to 3 can be freely combined with acontent described in this embodiment.

First, one example of the shift register is described with reference toFIG. 19. A shift register 400 includes a plurality of flip-flops offlip-flops 401_1 to 401_N (N is a natural number).

Note that the flip-flops 401_1 to 401_N each correspond to thesemiconductor device described in Embodiments 1 to 3. One example inFIG. 19 shows the case where the semiconductor device in FIG. 1A is usedas each of the flip-flops 401_1 to 401N. However, this embodiment is notlimited to this example. As the flip-flops 401_1 to 401_N, thesemiconductor device described in Embodiments 1 to 3 or a variety ofsemiconductor devices or circuits can be used other than thesemiconductor device in FIG. 1A.

Next, connection relations of the shift register are described. Theshift register 400 is connected to wirings 411_1 to 411_N, a wiring 412,a wiring 413, a wiring 414, and a wiring 415. In addition, in theflip-flop 401 _(—) i (i is any one of 1 to N), the wiring 111 isconnected to the wiring 411 _(—) i, the wiring 112 is connected to oneof the wirings 412 and 413, the wiring 113 is connected to the other ofthe wirings 412 and 413, the wiring 114 is connected to the wiring 411_(—) i−1, the wiring 115 is connected to the wiring 411 _(—) i+1, andthe wiring 116 is connected to the wiring 416. Here, the wirings towhich the wiring 112 and the wiring 113 are connected in flip-flops ofthe odd-numbered stages are reversed in flip-flops of the even-numberedstages in many cases. For example, in the case where the wirings 112 areconnected to the wiring 412 and the wirings 113 are connected to thewiring 413 in the flip-flops of the odd-numbered stages, the wirings 112are connected to the wiring 413 and the wirings 113 are connected to thewiring 412 in the flip-flops of the even-numbered stages in many cases.On the other hand, in the case where the wirings 112 in the flip-flopsof the odd-numbered stages are connected to the wiring 413 and thewirings 113 in the flip-flops of the odd-numbered stages are connectedto the wiring 412, the wirings 112 in the flip-flops of theeven-numbered stages are connected to the wiring 412 and the wirings 113in the flip-flops of the even-numbered stages are connected to thewiring 413 in many cases. However, this embodiment is not limited tothis example. A variety of connection relations other than above can beemployed.

Note that the wiring 114 in the flip-flop 401_1 is connected to thewiring 414 in many cases. In addition, the wiring 115 in the flip-flop401_N is connected to the wiring 415 in many cases.

Note that the wirings 411_1 to 411_N each correspond to the wiring 111described in Embodiments 1 to 3. The wiring 412 corresponds to thewiring 112 or the wiring 113 described in Embodiments 1 to 3. The wiring413 corresponds to the wiring 112 or the wiring 113 described inEmbodiments 1 to 3. The wiring 414 corresponds to the wiring 114described in Embodiments 1 to 3. The wiring 415 corresponds to thewiring 115 described in Embodiments 1 to 3. The wiring 416 correspondsto the wiring 116 described in Embodiments 1 to 3.

Next, one example of a signal or voltage input or output to/from each ofthe wirings 411_1 to 411_N, the wiring 412, the wiring 413, the wiring414, the wiring 415, and the wiring 416 is described. For example,signals GOUT_1 to GOUT_N are output from the wirings 411_1 to 411_N,respectively. The signals GOUT_1 to GOUT_N are output signals from theflip-flops 401_1 to 401_N, respectively. In addition, the signals GOUT_1to GOUT_N each correspond to the signal OUT described in Embodiments 1to 3 and each can function as an output signal, a selection signal, atransfer signal, a start signal, a reset signal, a gate signal, or ascan signal. For example, a signal GCK is input to the wiring 412. Thesignal GCK corresponds to the signal IN1 or the signal IN2 described inEmbodiments 1 to 3 and can function as a clock signal. For example, asignal GCKB is input to the wiring 413. The signal GCKB corresponds tothe signal IN1 or the signal IN2 described in Embodiments 1 to 3 and canfunction as an inverted clock signal. For example, a signal GSP is inputto the wiring 414. The signal GSP corresponds to the signal IN3described in Embodiments 1 to 3 and can function as a start signal or avertical synchronization signal. For example, a signal GRE is input tothe wiring 415. The signal GRE corresponds to the signal IN4 describedin Embodiments 1 to 3 and can function as a reset signal. For example,the voltage V1 is input to the wiring 416. However, this embodiment isnot limited to this example. A variety of signals, currents, or voltagesother than above can be input to each of the wirings 411_1 to 411_N, thewiring 412, the wiring 413, the wiring 414, the wiring 415, and/or thewiring 416. For example, voltage such as the voltage V1 or the voltageV2 can be supplied to the wiring 412, the wiring 413, the wiring 414,and/or the wiring 415. Alternatively, a signal such as the signals GOUT1 to GOUT_N, the signal GCK, the signal GCKB, the signal GSP, or thesignal GRE can be input to the wiring 416. Alternatively, the wirings411_1 to 411_N, the wiring 412, the wiring 413, the wiring 414, thewiring 415, and/or the wiring 416 can be made to go into a floatingstate without input of a signal, voltage, or the like.

Note that the wirings 411_1 to 411_N can each function as a signal line,a gate line, a scan line, or an output signal line. The wiring 412 canfunction as a signal line or a clock signal line. The wiring 413 canfunction as a signal line or a clock signal line. The wiring 414 canfunction as a signal line. The wiring 415 can function as a signal line.The wiring 416 can function as a power supply line or a ground line.However, this embodiment is not limited to this example. The wirings411_1 to 411_N, the wiring 412, the wiring 413, the wiring 414, thewiring 415, and/or the wiring 416 can function as a variety of wiringsother than above. For example, in the case where voltage is supplied tothe wiring 412, the wiring 413, the wiring 414, and/or the wiring 415,these wirings can function as power supply lines. Alternatively, in thecase where a signal is input to the wiring 416, the wiring 416 canfunction as a signal line.

Note that as described above, polyphase clock signals or unbalancedclock signals can be input to the shift register.

Note that a signal, voltage, or the like is input to the wiring 412, thewiring 413, the wiring 414, the wiring 415, and the wiring 416 from acircuit 420. The circuit 420 has a function of controlling the shiftregister 400 by supplying a signal, voltage, or the like to the shiftregister 400 and can function as a control circuit, a controller, or thelike. In this embodiment, for example, the circuit 420 supplies thesignal GCK, the signal GCKB, the signal GSP, the signal GRE, and thevoltage V1 to the wiring 412, the wiring 413, the wiring 414, the wiring415, and the wiring 416, respectively. However, this embodiment is notlimited to this example. The circuit 420 can control not only the shiftregister 400 but also a variety of circuits (e.g., a signal line drivercircuit, a scan line driver circuit, and/or a pixel) other than above bysupplying a signal or voltage to the circuit.

Note that the circuit 420 includes a circuit 421 and a circuit 422, forexample. The circuit 421 has a function of generating power supplyvoltage such as positive power supply voltage, negative power supplyvoltage, ground voltage, or reference voltage and can function as apower supply circuit or a regulator. The circuit 422 has a function ofgenerating a variety of signals such as a clock signal, an invertedclock signal, a start signal, a reset signal, and/or a video signal andcan function as a timing generator. However, this embodiment is notlimited to this example. The circuit 420 can include a variety ofcircuits or elements other than the circuit 421 and the circuit 422. Forexample, the circuit 420 can include an oscillator, a level shiftcircuit, an inverter circuit, a buffer circuit, a DA converter circuit,an AD converter circuit, an operational amplifier, a shift register, alook-up table, a coil, a transistor, a capacitor, a resistor, afrequency divider, and/or the like.

Next, operation of the shift register in FIG. 19 is described withreference to a timing chart in FIG. 20. FIG. 20 is one example of atiming chart for description of the operation of the shift register.FIG. 20 shows one example of each of the signal GSP, the signal GRE, thesignal GCK, the signal GCKB, the signal GOUT_i, the signal GOUT_i−1, thesignal GOUT_i, the signal GOUT_i+1, and the signal GOUT_N. Note thatdescription of operation which is the same as that of the semiconductordevice in Embodiments 1 to 3 is omitted.

Operation of the flip-flop 401 _(—) i is described. First, the signalGOUT_i−1 goes into an H level. Then, the flip-flop 401 _(—) i startsoperation of a period T1 and the signal GOUT_i goes into an L level.After that, the signal GCK and the signal GCKB are inverted. Then, theflip-flop 401 _(—) i starts operation of a period T2 and the signalGOUT_i goes into an H level. The signal GOUT_i is input to the flip-flop401 _(—) i−1 as a reset signal and input to the flip-flop 401 _(—) i+1as a start signal. Therefore, the flip-flop 401 _(—) i−1 startsoperation of a period T3 and the flip-flop 401 _(—) i+1 starts operationof the period T1. After that, the signal GCK and the signal GCKB areinverted again. Then, the flip-flop 401 _(—) i+1 starts operation of theperiod T2 and the signal GOUT_i+1 goes into an H level. The signalGOUT_i+1 is input to the flip-flop 401 _(—) i as a reset signal.Accordingly, since the flip-flop 401 _(—) i starts operation of theperiod T3, the signal GOUT_i goes into an L level. After that, until thesignal GOUT_i−1 goes into the H level again, the flip-flop 401 _(—) ialternately perform operations of a period T4 and a period T5 every timethe signal GCK and the signal GCKB are inverted.

Note that the signal GSP is input to the flip-flop 401_1 from thecircuit 420 through the wiring 414 instead of an output signal from aflip-flop of the previous stage. Therefore, when the signal GSP goesinto an H level, the flip-flop 401_1 starts operation of the period T1.

Note that the signal GRE is input to the flip-flop 401_N from thecircuit 420 through the wiring 415 instead of an output signal from aflip-flop of the next stage. Therefore, when the signal GRE goes into anH level, the flip-flop 401_N starts operation of the period T3.

The above is the description of the operation of the shift register inthis embodiment. By using the semiconductor device in Embodiments 1 to3, the shift register in this embodiment can obtain a similar advantageto the semiconductor device.

Note that as described in Embodiments 1 to 3, the relation between thesignal GCK and the signal GCKB can be unbalanced. For example, as shownin a timing chart in FIG. 21A, a period in which the signal GCK and thesignal GCKB are in an H level can be shorter than a period in which thesignal GCK and the signal GCKB are in in an L level. In this manner,even if delay or distortion occurs in the signals GOUT_1 to GOUT_N,these signals can be prevented from going into a period of an H level.Therefore, in the case where the shift register in this embodiment isused for a display device, selection of a plurality of rows at one timecan be prevented. However, this embodiment is not limited to thisexample. The period in which the signal GCK and/or the signal GCKB arein the H level can be longer than the period in which the signal GCKand/or the signal GCKB are in the L level.

Note that as described in Embodiments 1 to 3, polyphase clock signalscan be used. For example, as shown in a timing chart in FIG. 21B,M-phase (M is a natural number) clock signals can be used. In that case,periods in which the signals GOUT_1 to GOUT_N are in an H level in onestage can overlap with periods in which the signals GOUT_1 to GOUT_N arein an H level in the previous stage and the next stage. Accordingly, inthe case where this embodiment is applied to a display device, aplurality of rows is selected at one time. Therefore, a video signal forpixels in a different row can be used as precharge voltage.

Note that in FIG. 21B, it is preferable that M is less than or equal to8. More preferably, M is less than or equal to 6. Further preferably, Mis less than or equal to 4. This is because, in the case where the shiftregister is used for a scan line driver circuit of a display device, ifM is too large a number, a plural kinds of video signals are written toa pixel; then, since a period in which a wrong video signal is writtento the pixel becomes long, display quality deteriorates in some cases.

Note that like in FIG. 21B, polyphase clock signals can be used in thetiming chart in FIG. 21A.

Note that the wiring 415 and another wiring (e.g., the wiring 412, thewiring 413, the wiring 414, or the wiring 416) can be brought togetherinto one wiring; that is, the number of wirings can be reduced. In thatcase, the wiring 415 is eliminated and the wiring 115 in the flip-flop401_N can be connected to the wiring 412, the wiring 413, the wiring414, or the wiring 416. In another example, the wiring 415 can beeliminated. In that case, like in FIG. 14B, the transistor 303 and thetransistor 304 included in the circuit 105 in the flip-flop 401_N can beeliminated.

Note that a wiring can be additionally provided depending on thestructure of the flip-flops 401_1 to 401_N. For example, in the casewhere a signal which can function as the voltage V2 or an all-stagesreset signal is required like in FIG. 17A or 17B, a wiring can beadditionally provided. In addition, a signal, voltage, or the like canbe supplied to the additionally provided wiring from the circuit 420.

Note that as shown in FIG. 22, a transistor 431 can be added to each ofthe flip-flops 401_1 to 401_N. The transistor 431 preferably has thesame polarity as the transistor 101 and is an n-channel transistor inmany cases. However, this embodiment is not limited to this example. Thetransistor 431 can be a p-channel transistor. In the flip-flop 401 _(—)i, a first terminal of the transistor 431 is connected to the wiring112, a second terminal of the transistor 431 is connected to a wiring417_4 and a gate of the transistor 431 is connected to the node A.Further, the wiring 111 in the flip-flop 401 _(—) i is connected to thewiring 411_4 the wiring 112 in the flip-flop 401 _(—) i is connected toone of the wiring 412 and the wiring 413, the wiring 113 in theflip-flop 401 _(—) i is connected to the other of the wiring 412 and thewiring 413. The wiring 114 in the flip-flop 401 _(—) i is connected to awiring 417 _(—) i−1. The wiring 115 in the flip-flop 401 _(—) i isconnected to the wiring 411 _(—) i+1. The wiring 116 in the flip-flop401 _(—) i is connected to the wiring 416. Accordingly, even in the casewhere a load such as a pixel or a gate line is connected to the wirings411_1 to 411_N, distortion, delay, or the like does not occur in atransfer signal for driving a flip-flop of the next stage. Therefore,adverse effect due to delay of the shift register can be suppressed.However, this embodiment is not limited to this example. The wiring 114can be connected to the wiring 411 _(—) i−1. Alternatively, the wiring115 can be connected to a wiring 417 _(—) i+1. Alternatively, atransistor for maintaining the potential of wirings 417_1 to 417N as V1can be additionally provided.

Note that in FIG. 22, the signal GCK and the signal GCKB can beunbalanced like in FIG. 21A. Alternatively, like in FIG. 21B, polyphaseclock signals can be used.

Embodiment 5

In this embodiment, an example of a display device is described.

First, an example of a system block of a liquid crystal display deviceis described with reference to FIG. 23A. The liquid crystal displaydevice includes a circuit 5361, a circuit 5362, a circuit 5363_1, acircuit 5363_2, a pixel portion 5364, a circuit 5365, and a lightingdevice 5366. A plurality of wirings 5371 which are extended from thecircuit 5362 and a plurality of wirings 5372 which are extended from thecircuit 5363_1 and the circuit 5363_2 are provided in the pixel portion5364. In addition, pixels 5367 which include display elements such asliquid crystal elements are provided in a matrix in respective regionswhere the plurality of wirings 5371 and the plurality of wirings 5372intersect with each other.

The circuit 5361 has a function of supplying a signal, voltage, current,or the like to the circuit 5362, the circuit 5363_1, the circuit 5363_2,and the circuit 5365 in response to a video signal 5360 and can serve asa controller, a control circuit, a timing generator, a power supplycircuit, a regulator, or the like. In this embodiment, for example, thecircuit 5361 supplies a signal line driver circuit start signal (SSP), asignal line driver circuit clock signal (SCK), an inverted signal linedriver circuit clock signal (SCKB), video signal data (DATA), or a latchsignal (LAT) to the circuit 5362. Alternatively, for example, thecircuit 5361 supplies a scan line driver circuit start signal (GSP), ascan line driver circuit clock signal (GCK), or an inverted scan linedriver circuit clock signal (GCKB) to the circuit 5363_1 and the circuit5363_2. Alternatively, the circuit 5361 supplies a backlight controlsignal (BLC) to the circuit 5365. Note that this embodiment is notlimited to this example. The circuit 5361 can supply a variety ofsignals, voltages, currents, or the like to the circuit 5362, thecircuit 5363_1, the circuit 5363_2, and the circuit 5365.

The circuit 5362 has a function of outputting video signals to theplurality of wirings 5371 in response to a signal supplied from thecircuit 5361 (e.g., SSP, SCK, SCKB, DATA, or LAT) and can serve as asignal line driver circuit. The circuit 5363_1 and the circuit 5363_2each have a function of outputting scan signals to the plurality ofwirings 5372 in response to a signal supplied from the circuit 5361(e.g., GSP, GCK, or GCKB) and can serve as a scan line driver circuit.The circuit 5365 has a function of controlling the luminance (or averageluminance) of the lighting device 5366 by controlling the amount ofelectric power supplied to the lighting device 5366, time to supply theelectric power to the lighting device 5366, or the like in response tothe signal (BLC) supplied from the circuit 5361 and can serve as a powersupply circuit.

Note that in the case where video signals are input to the plurality ofwirings 5371, the plurality of wirings 5371 can serve as signal lines,video signal lines, source lines, or the like. In the case where scansignals are input to the plurality of wirings 5372, the plurality ofwirings 5372 can serve as signal lines, scan lines, gate lines, or thelike. Note that one example of this embodiment is not limited to thisexample.

Note that in the case where the same signal is input to the circuit5363_1 and the circuit 5363_2 from the circuit 5361, scan signals outputfrom the circuit 5363_1 to the plurality of wirings 5372 and scansignals output from the circuit 5363_2 to the plurality of wirings 5372have approximately the same timings in many cases. Therefore, loadcaused by driving of the circuit 5363_1 and the circuit 5363_2 can bereduced. Accordingly, the display device can be made larger.Alternatively, the display device can have higher definition.Alternatively, since the channel width of transistors included in thecircuit 5363_1 and the circuit 5363_2 can be reduced, a display devicewith a narrower frame can be obtained. Note that this embodiment is notlimited to this example. The circuit 5361 can supply different signalsto the circuit 5363_1 and the circuit 53632.

Note that one of the circuit 5363_1 and the circuit 53632 can beeliminated.

Note that a wiring such as a capacitor line, a power supply line, or ascan line can be additionally provided in the pixel portion 5364. Then,the circuit 5361 can output a signal, voltage, or the like to such awiring. Alternatively, a circuit which is similar to the circuit 5363_1or the circuit 5363_2 can be additionally provided. The additionallyprovided circuit Can output a signal such as a scan signal to theadditionally provided wiring.

Note that the pixel 5367 can include a light-emitting element such as anEL element as a display element. In this case, as illustrated in FIG.23B, since the display element can emit light, the circuit 5365 and thelighting device 5366 can be eliminated. In addition, in order to supplyelectric power to the display element, a plurality of wirings 5373 whichcan serve as power supply lines can be provided in the pixel portion5364. The circuit 5361 can supply power supply voltage called voltage(ANO) to the wirings 5373. The wirings 5373 can be separately connectedto the pixels in accordance with color elements or connected to all thepixels.

Note that FIG. 23B illustrates an example in which the circuit 5361supplies different signals to the circuit 5363_1 and the circuit 5363_2.The circuit 5361 supplies a signal such as a scan line driver circuitstart signal (GSP1), a scan line driver circuit clock signal (GCK1), oran inverted scan line driver circuit clock signal (GCKB1) to the circuit5363_1. In addition, the circuit 5361 supplies a signal such as a scanline driver circuit start signal (GSP2), a scan line driver circuitclock signal (GCK2), or an inverted scan line driver circuit clocksignal (GCKB2) to the circuit 5363_2. In this case, the circuit 5363_1can scan only wirings in odd-numbered rows of the plurality of wirings5372 and the circuit 5363_2 can scan only wirings in even-numbered rowsof the plurality of wirings 5372. Thus, the driving frequency of thecircuit 5363_1 and the circuit 5363_2 can be lowered, so that powerconsumption can be reduced. Alternatively, an area in which a flip-flopof one stage can be laid out can be made larger. Therefore, a displaydevice can have higher definition. Alternatively, a display device canbe made larger. Note that this embodiment is not limited to thisexample. As in FIG. 23A, the circuit 5361 can supply the same signal tothe circuit 5363_1 and the circuit 5363_2.

Note that as in FIG. 23B, the circuit 5361 can supply different signalsto the circuit 5363_1 and the circuit 5363_2 in FIG. 23A.

Thus far, the example of a system block of a display device isdescribed.

Next, examples of structures of the display devices are described withreference to FIGS. 24A to 24E.

In FIG. 24A, circuits which have a function of outputting signals to thepixel portion 5364 (e.g., the circuit 5362, the circuit 5363_1, and thecircuit 5363_2) are Mimed over the same substrate 5380 as the pixelportion 5364. In addition, the circuit 5361 is formed over a differentsubstrate from the pixel portion 5364. In this manner, since the numberof external components is reduced, reduction in cost can be achieved.Alternatively, since the number of signals or voltages input to thesubstrate 5380 is reduced, the number of connections between thesubstrate 5380 and the external component can be reduced. Therefore,improvement in reliability or the increase in yield can be achieved.

Note that in the case where the circuit is formed over a differentsubstrate from the pixel portion 5364, the substrate can be mounted onan FPC (flexible printed circuit) by TAB (tape automated bonding).Alternatively, the substrate can be mounted on the same substrate 5380as the pixel portion 5364 by COG (chip on glass).

Note that in the case where the circuit is formed over a differentsubstrate from the pixel portion 5364, a transistor formed using asingle crystal semiconductor can be formed on the substrate. Therefore,the circuit formed over the substrate can have advantages such asimprovement in driving frequency, improvement in driving voltage, andsuppression of variations in output signals.

Note that a signal, voltage, current, or the like is input from anexternal circuit through an input terminal 5381 in many cases.

In FIG. 24B, circuits with low driving frequency (e.g., the circuit5363_1 and the circuit 5363_2) are fanned over the same substrate 5380as the pixel portion 5364. In addition, the circuit 5361 and the circuit5362 are formed over a different substrate from the pixel portion 5364.In this manner, since the circuit formed over the substrate 5380 can beformed using a transistor with low mobility, a non-single-crystalsemiconductor, an amorphous semiconductor, a microcrystallinesemiconductor, an organic semiconductor, an oxide semiconductor, or thelike can be used for a semiconductor layer of the transistor.Accordingly, the increase in the size of the display device, reductionin the number of steps, reduction in cost, improvement in yield, or thelike can be achieved.

Note that as illustrated in FIG. 24C, part of the circuit 5362 (acircuit 5362 a) can be formed over the same substrate 5380 as the pixelportion 5364 and the other part of the circuit 5362 (a circuit 5362 b)can be formed over a different substrate from the pixel portion 5364.The circuit 5362 a includes a circuit which can be formed using atransistor with low mobility (e.g., a shift register, a selector, or aswitch) in many cases. In addition, the circuit 5362 b includes acircuit which is preferably formed using a transistor with high mobilityand few variations in characteristics (e.g., a shift register, a latchcircuit, a buffer circuit, a DA converter circuit, or an AD convertercircuit) in many cases. In this manner, as in FIG. 24B, anon-single-crystal semiconductor, an amorphous semiconductor, amicrocrystalline semiconductor, an organic semiconductor, an oxidesemiconductor, or the like can be used for a semiconductor layer of thetransistor. Further, reduction in external components can be achieved.

In FIG. 24D, circuits which have a function of outputting signals to thepixel portion 5364 (e.g., the circuit 5362, the circuit 5363_1, and thecircuit 5363_2) and a circuit which has a function of controlling thesecircuits (e.g., the circuit 5361) are formed over a different substratefrom the pixel portion 5364. In this manner, since the pixel portion andperipheral circuits thereof can be formed over different substrates,improvement in yield can be achieved.

Note that as in FIG. 24D, the circuit 5363_1 and the circuit 5363_2 canbe formed over a different substrate from the pixel portion 5364 inFIGS. 24A to 24C.

In FIG. 24E, part of the circuit 5361 (a circuit 5361 a) is formed overthe same substrate 5380 as the pixel portion 5364 and the other part ofthe circuit 5361 (a circuit 5361 b) is formed over a different substratefrom the pixel portion 5364. The circuit 5361 a includes a circuit whichcan be formed using a transistor with low mobility (e.g., a switch, aselector, or a level shift circuit) in many cases. In addition, thecircuit 5361 b includes a circuit which is preferably formed using atransistor with high mobility and few variations (e.g., a shiftregister, a timing generator, an oscillator, a regulator, or an analogbuffer) in many cases.

Note that also in FIGS. 24A to 24D, the circuit 5361 a can be formedover the same substrate as the pixel portion 5364 and the circuit 5361 bcan be formed over a different substrate from the pixel portion 5364.

The above is the description of the display device in this embodiment.As each of the circuit 5363_1 and the circuit 5363_2, the semiconductordevice or the shift register in Embodiments 1 to 4 can be used. In thatcase, since the circuit 5363_1, the circuit 5363_2, and the pixelportion are formed over one substrate, all the transistors formed overthe substrate can be n-channel transistors or all the transistors formedover the substrate can be p-channel transistors. Accordingly, reductionin the number of steps, improvement in yield, improvement inreliability, or reduction in cost can be achieved. Specifically, if allthe transistors are n-channel transistors, non-single-crystalsemiconductors, microcrystalline semiconductors, organic semiconductors,oxide semiconductors, or the like can be used for semiconductor layersof the transistors. Accordingly, increase in the size of the displaydevice, reduction in cost, improvement in yield, or the like can beachieved.

Note that deterioration of characteristics such as increase in thresholdvoltage or decrease in mobility is caused in a transistor in which anon-single-crystal semiconductor, a microcrystalline semiconductor, anorganic semiconductor, an oxide semiconductor, or the like is used as asemiconductor layer in many cases. However, since deterioration ofcharacteristics of the transistor in the semiconductor device or theshift register in Embodiments 1 to 4 can be suppressed, the life of adisplay device can be made longer.

Note that as part of the circuit 5362, the semiconductor device or theshift register in Embodiments 1 to 4 can be used. For example, thecircuit 5362 a can include the semiconductor device or the shiftregister in Embodiments 1 to 4.

Embodiment 6

In this embodiment, one example of a signal line driver circuit will bedescribed. Note that the signal line driver circuit can be referred toas a semiconductor device or a signal generation circuit.

One example of the signal line driver circuit is described withreference to FIG. 25A. The signal line driver circuit includes aplurality of circuits of circuits 502_1 to 502_N (N is a naturalnumber), a circuit 500, and a circuit 501. In addition, the circuits502_1 to 502_N each include a plurality of transistors of transistors503_1 to 503 _(—) k (k is a natural number). The transistors 503_1 to503 _(—) k are n-channel transistors. However, this embodiment is notlimited to this example. The transistors 503_1 to 503 _(—) k can bep-channel transistors or CMOS switches.

A connection relation of the signal line driver circuit will bedescribed by using the circuit 502_1 as an example. First terminals ofthe transistors 503_1 to 503 _(—) k are connected to the wiring 505_1.Second terminals of the transistors 503_1 to 503 _(—) k are connected towirings S1 to Sk, respectively. Gates of the transistors 503_1 to 503_(—) k are connected to wirings 504_1 to 504 _(—) k, respectively. Forexample, the first terminal of the transistor 503_1 is connected to thewiring 505_1, the second terminal of the transistor 503_1 is connectedto the wiring S1, and the gate of the transistor 503_1 is connected tothe wiring 504_1.

The circuit 500 has a function of supplying a signal to the circuits502_1 to 502_N through the wirings 504_1 to 504 _(—) k and can functionas a shift register, a decoder, or the like. The signal is a digitalsignal in many cases and can function as a selection signal. Inaddition, the wirings 504_1 to 504 _(—) k can function as signal lines.

The circuit 501 has a function of outputting a signal to the circuits502_1 to 502_N and can function as a video signal generation circuit orthe like. For example, the circuit 501 supplies the signal to thecircuit 502_1 through the wiring 505_1. At the same time, the circuit501 supplies the signal to the circuit 502_2 through the wiring 505_2.The signal is an analog signal in many cases and can function as a videosignal. In addition, the wirings 505_1 to 505_N can function as signallines.

The circuits 502_1 to 502_N each have a function of selecting a wiringto which an output signal from the circuit 501 is output and canfunction as a selector circuit. For example, the circuit 502_1 has afunction of selecting one of the wirings S1 to Sk to output a signal tobe output from the circuit 501 to the wiring 505_1.

The transistors 503_1 to 503 _(—) k have a function of controlling theelectric conduction state between the wiring 505_1 and the wirings S1 toSk in accordance with the output signal from the circuit 500 andfunction as switches.

Next, operation of the signal line driver circuit shown in FIG. 25A isdescribed with reference to a timing chart in FIG. 25B. FIG. 25B showsexamples of a signal 514_1 input to the wiring 504_1, a signal 514_2input to the wiring 5042, a signal 514 _(—) k input to the wiring 504_(—) k, a signal 515_1 input to the wiring 505_1, and a signal 515_2input to the wiring 505_2.

Note that one operation period of the signal line driver circuitcorresponds to one gate selection period in a display device. One gateselection period is a period in which a pixel which belongs to one rowis selected and a video signal can be written to the pixel.

Note that one gate selection period is divided into a period T0 and aperiod T1 to a period Tk. The period T0 is a period for applying voltagefor precharge on pixels which belong to a selected row at the same timeand can be used as a precharge period. Each of the periods T1 to Tk is aperiod in which a video signal is written to pixels which belong to theselected row and can be used as a writing period.

Note that for simplicity, operation of the signal line driver circuit isdescribed by using operation of the circuit 502_1 as an example.

First, during the period T0, the circuit 500 outputs a signal in an Hlevel to the wirings 504_1 to 504 k. Then, the transistors 503_1 to 503_(—) k are turned on, whereby the wiring 505_1 and the wirings S1 to Skare brought into electrical conduction. At that time, the circuit 501supplies precharge voltage Vp to the wiring 505_1, so that the prechargevoltage Vp is output to the wirings S1 to Sk through the transistors503_1 to 503 _(—) k, respectively. Then, the precharge voltage Vp iswritten to the pixels which belong to the selected row, whereby thepixels which belong to the selected row are precharged.

Next, during the period T1, the circuit 500 outputs a signal in an Hlevel to the wirings 504_1. Then, the transistor 503_1 is turned on,whereby the wiring 505_1 and the wiring S1 are brought into electricalconduction. Then, the wiring 505_1 and the wirings S2 to Sk are broughtout of electrical conduction. At that time, the circuit 501 supplies asignal Data (S1) to the wiring 505_1, so that the signal Data (S1) isoutput to the wiring S1 through the transistors 503_1. In this manner,the signal Data (S1) is written to, of the pixels connected to thewiring S1, the pixels which belong to the selected row.

Next, during the period T2, the circuit 500 outputs a signal in an Hlevel to the wirings 504_2. Then, the transistor 503_2 is turned on,whereby the wiring 505_2 and the wiring S2 are brought into electricalconduction. Then, the wiring 505_1 and the wirings S1 are brought out ofelectrical conduction while the wiring 505_1 and the wirings S3 to Skare kept out of electrical conduction. At that time, when the circuit501 outputs a signal Data (S2) to the wiring 505_1, the signal Data (S2)is output to the wiring S2 through the transistors 503_2. In thismanner, the signal Data (S2) is written to, of the pixels connected tothe wiring S2, the pixels which belong to the selected row.

After that, since the circuit 500 sequentially outputs signals in an Hlevel to the wirings 504_1 to 504 _(—) k until the end of the period Tk,the circuit 500 sequentially outputs the signal in the H level to thewirings 504_3 to 504 _(—) k from the period T3 to the period Tk, as inthe period T1 and the period T2. Therefore, since the transistors 503_3to 503 _(—) k are sequentially turned on, the transistors 503_1 to 503_(—) k are sequentially turned on. Accordingly, a signal output from thecircuit 501 is sequentially output to the wirings S1 to Sk. In thismanner, the signal can be sequentially written to the pixels whichbelong to the selected row.

The above is the description of the one example of the signal linedriver circuit. Since the signal line driver circuit in this embodimentincludes the circuit which functions as a selector, the number ofsignals or wirings can be reduced. Alternatively, since voltage forprecharging is written to the pixel before a video signal is written tothe pixel (during period T0), a writing time of the video signal can beshortened. Accordingly, increase in the size of a display device andhigher resolution of the display device can be achieved. However, thisembodiment is not limited to this example. It is acceptable that theperiod T0 is eliminated, so that the pixel is not precharged.

Note that if k is too large a number, a writing time of the pixel isshortened, whereby writing of a video signal to the pixel is notcompleted in the writing time in some cases. Accordingly, it ispreferable that k≦6. It is more preferable that k≦3. It is furtherpreferable that k=2.

Specifically, in the case where a color element of a pixel is dividedinto n (n is a natural number), it can be acceptable that k=n. Forexample, in the case where a color element of a pixel is divided intored (R), green (G), and blue (B), it can be acceptable that k=3. In thatcase, one gate selection period is divided into a period T0, a periodT1, a period T2, and a period T3. Then, a video signal can be written tothe pixel of red (R), the pixel of green (G), and the pixel of blue (B)during the period T1, the period T2, and the period T3, respectively.However, this embodiment is not limited to this example. The order ofthe period T1, the period T2, and the period T3 can be appropriatelyset.

Specifically, in the case where a pixel is divided into n (n is anatural number) sub-pixels, it can be acceptable that k=n. For example,in the case where the pixel is divided into two sub-pixels, it can beacceptable that k=2. In that case, one gate selection period is dividedinto the period T0, the period T1, and the period T2. Then, a videosignal can be written to one of the two sub-pixels during the period T1,and a video signal can be written to the other of the two sub-pixelsduring the period T2.

Note that since the driving frequencies of the circuit 500 and thecircuits 502_1 to 502_N are low in many cases, the circuit 500 and thecircuits 502_1 to 502_N can be formed over the same substrate as a pixelportion. In this manner, since the number of connections between thesubstrate over which the pixel portion is formed and an external circuitcan be reduced, increase in yield, improvement in reliability, or thelike can be achieved. Further, as shown in FIG. 24C, by also forming ascan line driver circuit over the same substrate as the pixel portion,the number of connections between the substrate over which the pixelportion is formed and the external circuit can be further reduced.

Note that the semiconductor device or shift register described inEmbodiments 1 to 4 can be used as the circuit 500. In that case, alltransistors in the circuit 500 can be n-channel transistors or all thetransistors in the circuit 500 can be p-channel transistors.Accordingly, reduction in the number of steps, increase in yield, orreduction in cost can be achieved.

Note that not only all the transistors included in the circuit 500 butalso all transistors in the circuits 502_1 to 502_N can be n-channeltransistors or all the transistors in the circuits 502_1 to 502_N can bep-channel transistors. Therefore, in the case where the circuit 500 andthe circuits 502_1 to 502_N are formed over the same substrate as thepixel portion, reduction in the number of steps, increase in yield, orreduction in cost can be achieved. Specifically, by using n-channeltransistors as all the transistors, non-single-crystal semiconductors,amorphous semiconductors, microcrystalline semiconductors, organicsemiconductors, or oxide semiconductors can be used as semiconductorlayers of the transistors. This is because the driving frequencies ofthe circuit 500 and the circuits 502_1 to 502_N are low in many cases.

Embodiment 7

In this embodiment, a pixel structure and operation of a pixel which canbe applied to a liquid crystal display device are described.

FIG. 26A illustrates an example of a pixel. A pixel 5420 includes atransistor 5421, a liquid crystal element 5422, and a capacitor 5423. Afirst terminal of the transistor 5421 is connected to a wiring 5431, asecond terminal of the transistor 5421 is connected to one electrode ofthe liquid crystal element 5422 and one electrode of the capacitor 5423,and a gate of the transistor 5421 is connected to a wiring 5432. Theother electrode of the liquid crystal element 5422 is connected to anelectrode 5434 and the other electrode of the capacitor 5423 isconnected to a wiring 5433.

A video signal can be input to the wiring 5431, for example. A scansignal, a selection signal, or a gate signal can be input to the wiring5432, for example. A constant voltage can be supplied to the wiring5433, for example. A constant voltage can be supplied to the electrode5434, for example. However, this embodiment is not limited to thisexample. A writing time of a video signal can be shortened by supply ofprecharge voltage to the wiring 5431. Alternatively, voltage applied tothe liquid crystal element 5422 can be controlled by input of a signalto the wiring 5433. Alternatively, frame inversion driving can beachieved by input of a signal to the electrode 5434.

Note that the wiring 5431 can function as a signal line, a video signalline, or a source line. The wiring 5432 can function as a signal line, ascan line, or a gate line. The wiring 5433 can function as a powersupply line or a capacitor line. The electrode 5434 can function as acommon electrode or a counter electrode. However, this embodiment is notlimited to this example. In the case where voltage is supplied to thewiring 5431 and the wiring 5432, these wirings can function as powersupply lines. Alternatively, in the case where a signal is input to thewiring 5433, the wiring 5433 can function as a signal line.

The transistor 5421 has a function of controlling timing when a videosignal is written to a pixel by controlling the electrical conductionstate of the wiring 5431 and the one electrode of the liquid crystalelement 5422 and can function as a switch. The capacitor 5423 has afunction of keeping voltage applied to the liquid crystal element 5422as a stable value by storing the potential difference between the oneelectrode of the liquid crystal element 5422 and the wiring 5433 andfunctions as a storage capacitor. However, this embodiment is notlimited to this example.

FIG. 26B shows an example of a timing chart for illustrating theoperation of the pixel in FIG. 26A. FIG. 26B shows a signal 5442 _(—) j(j is a natural number), a signal 5442 _(—) j+1, a signal 5441 (i is anatural number), a signal 5441 _(—) j+1, and a voltage 5442. Inaddition, FIG. 26B shows a kth (k is a natural number) frame and a(k+1)th frame. Note that the signal 5442 _(—) j, the signal 5442 _(—)j+1, the signal 5441 _(—) i, the signal 5441 _(—) i+1, and the voltage5442 are examples of a signal input to the wiring 5432 in a jth row, asignal input to the wiring 5432 in a (j+1)th row, a signal input to thewiring 5431 in an ith column, a signal input to the wiring 5431 in an(i+1)th column, and voltage supplied to the wiring 5432, respectively.

Operation of the pixel 5420 in the jth row and the ith column isdescribed. When the signal 5442 _(—) j goes into an H level, thetransistor 5421 is turned on. Accordingly, since the wiring 5431 in theith column and the one electrode of the liquid crystal element 5422 arebrought into electrical conduction, the signal 5441 _(—) j is input tothe one electrode of the liquid crystal element 5422 through thetransistor 5421. Then, the capacitor 5423 keeps the potential differencebetween the one electrode of the liquid crystal element 5422 and thewiring 5433. Therefore, after that, voltage applied to the liquidcrystal element 5422 is constant until the signal 5422 _(—) j goes intothe H level again. Then, the liquid crystal element 5422 expresses graylevels corresponding to the applied voltage.

Note that FIG. 26B shows an example of the case where a positive signaland a negative signal are alternately input to the wiring 5431 every oneselection period. The positive signal is a signal whose potential ishigher than a reference value (e.g., the potential of the electrode5434). The negative signal is a signal whose potential is lower than areference value (e.g., the potential of the electrode 5434). However,this embodiment is not limited to this example. Signals with the samepolarity can be input to the wiring 5431 in one frame period.

Note that FIG. 26B shows an example of the case where the polarity ofthe signal 5441 _(—) i and the polarity of the signal 5441 _(—) i+1 aredifferent from each other. However, this embodiment is not limited tothis example. The polarity of the signal 5441 _(—) i and the polarity ofthe signal 5441 _(—) i+1 can be the same.

Note that FIG. 26B shows an example of the case where a period in whichthe signal 5442 j is in an H level and a period in which the signal 5442j+1 is in an H level do not overlap with each other. However, thisembodiment is not limited to this example. As shown in FIG. 26C, theperiod in which the signal 5442 j is in the H level and the period inwhich the signal 5442 _(—) j+1 is in the H level can overlap with eachother. In that case, signals of the same polarity are preferablysupplied to the wiring 5431 in one frame period. In this manner, pixelsin a (j+1)th row can be precharged by using the signal 5441 _(—) jwritten to pixels in the jth row. Accordingly, a writing time of a videosignal to a pixel can be shortened. Therefore, a high-definition displaydevice can be obtained. Alternatively, a display portion of the displaydevice can be made large. Alternatively, since the signals of the samepolarity are input to the wiring 5431 in one frame period, powerconsumption can be reduced.

Note that by a combination of a pixel structure in FIG. 27A and thetiming chart in FIG. 26C, dot inversion driving can be achieved. In thepixel structure in FIG. 27A, a pixel 5420(i, j) is connected to a wiring5431 _(—) i. On the other hand, a pixel 5420(i, j+1) is connected to awiring 5431 _(—) i+1. In other words, pixels in the ith column arealternately connected to the wiring 5431 _(—) i and the wiring 5431 _(—)i+1 row-by-row. In this manner, since a positive signal and a negativesignal are alternately written to the pixels in the ith columnrow-by-row, dot inversion driving can be achieved. However, thisembodiment is not limited to this example. The pixels, which are in theith column, of every plural rows (e.g., two rows or three rows) can bealternately connected to the wiring 5431 _(—) i and the wiring 5431 _(—)i+1.

Note that a sub-pixel structure can be used as the pixel structure.FIGS. 27B and 27C each show a structure in the case where a pixel isdivided into two sub-pixels. In addition, FIG. 27B shows a sub-pixelsstructure called 1S+2G and FIG. 27C shows a sub-pixel structure called2S+1G. A sub-pixel 5420A and a sub-pixel 5420B correspond to the pixel5420. A transistor 5421A and a transistor 5421B correspond to thetransistor 5421. A liquid crystal element 5422A and a liquid crystalelement 5422B correspond to the liquid crystal element 5422. A capacitor5423A and a capacitor 5423B correspond to the capacitor 5423. A wiring5431A and a wiring 5431B correspond to the wiring 5431. A wiring 5432Aand a wiring 5432B correspond to the wiring 5432.

The above is the description of the pixel structure and the drivingmethod of the pixel in this embodiment. By a combination of the pixel inthis embodiment and any of the semiconductor device, the shift register,the display device, and the signal line driver circuit which aredescribed in Embodiments 1 to 6, a variety of advantages can beobtained. For example, in the case where a sub-pixel structure isemployed for the pixel, the number of signals required for driving adisplay device is increased. Therefore, the number of gate lines orsource lines is increased. As a result, the number of connectionsbetween a substrate over which a pixel portion is formed and an externalcircuit is greatly increased in some cases. However, even if the numberof gate lines is increased, the scan line driver circuit can be formedover a substrate over which the pixel portion is formed, as described inEmbodiment 5. Therefore, the pixel with the sub-pixel structure can beused without greatly increasing the number of connections between thesubstrate over which the pixel portion is formed and the externalcircuit. Alternatively, even if the number of source lines is increased,the signal line driver circuit in Embodiment 6 can be formed over asubstrate over which the pixel portion is formed. Therefore, the pixelwith the sub-pixel structure can be used without greatly increasing thenumber of connections between the substrate over which the pixel portionis formed and the external circuit.

Alternatively, in the case where a signal is input to a capacitor line,the number of connections between the substrate over which the pixelportion is formed and the external circuit is greatly increased in somecases. For that case, a signal can be supplied to the capacitor line byusing any of the semiconductor device and the shift register inEmbodiments 1 to 4. In addition, the semiconductor device or the shiftregister in Embodiments 1 to 4 can be formed over the substrate overwhich the pixel portion is formed. Therefore, a signal can be input tothe capacitor line without greatly increasing the number of connectionsbetween the substrate over which the pixel portion is formed and theexternal circuit.

Alternatively, in the case where alternate-current driving is employed,a time for writing a video signal to the pixel is short. As a result,shortage of the time for writing the video signal to the pixel is causedin some cases. Similarly, in the case where the pixel with the sub-pixelstructure is used, the time for writing the video signal to the pixel isshort. As a result, shortage of the time for writing the video signal tothe pixel is caused in some cases. For that case, the video signal canbe written to the pixel by using the signal line driver circuit inEmbodiment 6. In that case, since voltage for precharge is written tothe pixel before the video signal is written to the pixel, the videosignal can be written to the pixel in a short time. Alternatively, asshown in FIG. 21B, by overlap of a period in which one row is selectedwith a period in which a different row is selected, a video signal forthe different row can be used as the voltage for precharge.

Embodiment 8

In this embodiment, an example of a cross-sectional structure of adisplay device will be described with reference to FIGS. 29A to 29C.

FIG. 29A illustrates an example of a top view of a display device. Adriver circuit 5392 and a pixel portion 5393 are formed over a substrate5391. An example of the driver circuit 5392 is a scan line drivercircuit or a signal line driver circuit.

FIG. 29B illustrates an example of the A-B cross section of FIG. 29A.FIG. 29B illustrates a substrate 5400, a conductive layer 5401 formedover the substrate 5400, an insulating layer 5402 formed so as to coverthe conductive layer 5401, a semiconductor layer 5403 a formed over theconductive layer 5401 and the insulating layer 5402, a semiconductorlayer 5403 b formed over the semiconductor layer 5403 a, a conductivelayer 5404 formed over the semiconductor layer 5403 b and the insulatinglayer 5402, an insulating layer 5405 formed over the insulating layer5402 and the conductive layer 5404 and having an opening portion, aconductive layer 5406 fowled over the insulating layer 5405 and in theopening portion in the insulating layer 5405, an insulating layer 5408provided over the insulating layer 5405 and the conductive layer 5406, aliquid crystal layer 5407 formed over the insulating layer 5405, aconductive layer 5409 formed over the liquid crystal layer 5407 and theinsulating layer 5408, and a substrate 5410 provided over the conductivelayer 5409.

The conductive layer 5401 can function as a gate electrode. Theinsulating layer 5402 can function as a gate insulating film. Theconductive layer 5404 can function as a wiring, an electrode of atransistor, an electrode of a capacitor, or the like. The insulatinglayer 5405 can function as an interlayer film or a planarization film.The conductive layer 5406 can function as a wiring, a pixel electrode,or a reflective electrode. The insulating layer 5408 can function as asealing material. The conductive layer 5409 can function as a counterelectrode or a common electrode.

Here, parasitic capacitance is sometimes generated between the drivercircuit 5392 and the conductive layer 5409. Thus, an output signal fromthe driver circuit 5392 or a potential of each node is distorted ordelayed, or power consumption is increased. However, when the insulatinglayer 5408 which can serve as the sealing material is formed over thedriver circuit 5392 as illustrated in FIG. 29B, parasitic capacitancegenerated between the driver circuit 5392 and the conductive layer 5409can be reduced. This is because the dielectric constant of the sealingmaterial is often lower than that of the liquid crystal layer.Accordingly, distortion or delay of the output signal from the drivercircuit 5392 or the potential of each node can be reduced.Alternatively, power consumption of the driver circuit 5392 can bereduced.

Note that as illustrated in FIG. 29C, the insulating layer 5408 whichcan function as the sealing material can be formed over part of thedriver circuit 5392. In such a case also, parasitic capacitancegenerated between the driver circuit 5392 and the conductive layer 5409can be reduced; thus, distortion or delay of the output signal from thedriver circuit 5392 or distortion or delay of the potential of each nodecan be reduced. Note that this embodiment is not limited thereto, and itis acceptable that the insulating layer 5408, which can function as thesealing material, is not formed over the driver circuit 5392.

Note that a display element is not limited to a liquid crystal element,and a variety of display elements such as an EL element or anelectrophoretic element can be used.

As above, this embodiment describes one example of the cross-sectionalstructure of the display device. Such a structure can be combined withthe semiconductor device or the shift register in Embodiments 1 to 4.For example, when a non-single-crystal semiconductor, an amorphoussemiconductor, a microcrystalline semiconductor, an organicsemiconductor, an oxide semiconductor, or the like is used for asemiconductor layer of a transistor, the channel width of the transistoris often increased. However, by reducing parasitic capacitance of thedriver circuit as in this embodiment, the channel width of thetransistor can be reduced. Accordingly, the layout area can be reduced,so that the frame of the display device can be reduced. Alternatively,the resolution of the display device can be increased.

Embodiment 9

In this embodiment, examples of structures of transistors are describedwith reference to FIGS. 30A, 30B, and 30C.

FIG. 30A illustrates an example of a structure of a top-gate transistor.FIG. 30B illustrates an example of a structure of a bottom-gatetransistor. FIG. 30C illustrates an example of a structure of atransistor fonued using a semiconductor substrate.

FIG. 30A illustrates a substrate 5260; an insulating layer 5261 formedover the substrate 5260; a semiconductor layer 5262 which is formed overthe insulating layer 5261 and is provided with a region 5262 a, a region5262 b, a region 5262 c, a region 5262 d, and a region 5262 e; aninsulating layer 5263 formed so as to cover the semiconductor layer5262; a conductive layer 5264 formed over the semiconductor layer 5262and the insulating layer 5263; an insulating layer 5265 which is formedover the insulating layer 5263 and the conductive layer 5264 and isprovided with openings; a conductive layer 5266 which is formed over theinsulating layer 5265 and in the openings formed in the insulating layer5265; an insulating layer 5267 which is formed over the conductive layer5266 and the insulating layer 5265 and is provided with an opening; aconductive layer 5268 which is formed over the insulating layer 5267 andin the opening formed in the insulating layer 5267; an insulating layer5269 which is formed over the insulating layer 5267 and the conductivelayer 5268 and is provided with the opening; a light-emitting layer 5270which is formed over the insulating layer 5269 and in the opening formedin the insulating layer 5269; and a conductive layer 5271 formed overthe insulating layer 5269 and the light-emitting layer 5270.

FIG. 30B illustrates a substrate 5300; a conductive layer 5301 formedover the substrate 5300; an insulating layer 5302 formed so as to coverthe conductive layer 5301; a semiconductor layer 5303 a formed over theconductive layer 5301 and the insulating layer 5302; a semiconductorlayer 5303 b formed over the semiconductor layer 5303 a; a conductivelayer 5304 formed over the semiconductor layer 5303 b and the insulatinglayer 5302; an insulating layer 5305 which is formed over the insulatinglayer 5302 and the conductive layer 5304 and is provided with anopening; a conductive layer 5306 which is formed over the insulatinglayer 5305 and in the opening formed in the insulating layer 5305; aliquid crystal layer 5307 formed over the insulating layer 5305 and theconductive layer 5306; and a conductive layer 5308 formed over theliquid crystal layer 5307.

FIG. 30C illustrates a semiconductor substrate 5352 including a region5353 and a region 5355; an insulating layer 5356 formed over thesemiconductor substrate 5352; an insulating layer 5354 formed over thesemiconductor substrate 5352; a conductive layer 5357 formed over theinsulating layer 5356; an insulating layer 5358 which is fowled over theinsulating layer 5354, the insulating layer 5356, and the conductivelayer 5357 and is provided with openings; and a conductive layer 5359which is formed over the insulating layer 5358 and in the openingsformed in the insulating layer 5358. Thus, a transistor is formed ineach of a region 5350 and a region 5351.

The insulating layer 5261 can serve as a base film. The insulating layer5354 serves as an element isolation layer (e.g., a field oxide film).Each of the insulating layer 5263, the insulating layer 5302, and theinsulating layer 5356 can serve as a gate insulating film. Each of theconductive layer 5264, the conductive layer 5301, and the conductivelayer 5357 can serve as a gate electrode. Each of the insulating layer5265, the insulating layer 5267, the insulating layer 5305, and theinsulating layer 5358 can serve as an interlayer film or a planarizationfilm. Each of the conductive layer 5266, the conductive layer 5304, andthe conductive layer 5359 can serve as a wiring, an electrode of atransistor, an electrode of a capacitor, or the like. Each of theconductive layer 5268 and the conductive layer 5306 can serve as a pixelelectrode, a reflective electrode, or the like. The insulating layer5269 can serve as a partition. Each of the conductive layer 5271 and theconductive layer 5308 can serve as a counter electrode, a commonelectrode, or the like.

As each of the substrate 5260 and the substrate 5300, a glass substrate,a quartz substrate, a silicon substrate (or a single crystal substrate),an SOI substrate, a plastic substrate, a metal substrate, a stainlesssteel substrate, a substrate including stainless steel foil, a tungstensubstrate, a substrate including tungsten foil, a flexible substrate, orthe like can be used, for example. As a glass substrate, a bariumborosilicate glass substrate, an aluminoborosilicate glass substrate, orthe like can be used, for example. For a flexible substrate, a flexiblesynthetic resin such as plastics typified by polyethylene terephthalate(PET), polyethylene naphthalate (PEN), and polyether sulfone (PES), oracrylic can be used, for example. Alternatively, an attachment film(formed using polypropylene, polyester, vinyl, polyvinyl fluoride,polyvinyl chloride, or the like), paper of a fibrous material, a basematerial film (formed using polyester, polyamide, an inorganic vapordeposition film, paper, or the like), or the like can be used.

As the semiconductor substrate 5352, for example, a single crystalsilicon substrate having n-type or p-type conductivity can be used. Notethat this embodiment is not limited to this example, and a substratewhich is similar to the substrate 5260 can be used. For example, theregion 5353 is a region where an impurity is added to the semiconductorsubstrate 5352 and serves as a well. For example, in the case where thesemiconductor substrate 5352 has p-type conductivity, the region 5353has n-type conductivity and serves as an n-well. On the other hand, inthe case where the semiconductor substrate 5352 has n-type conductivity,the region 5353 has p-type conductivity and serves as a p-well. Forexample, the region 5355 is a region where an impurity is added to thesemiconductor substrate 5352 and serves as a source region or a drainregion. Note that an LDD region can be formed in the semiconductorsubstrate 5352.

For the insulating layer 5261, an insulating film or a layered structureof an insulating film containing oxygen or nitrogen, such as siliconoxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y) canbe used, for example. In an example in the case where the insulatingfilm 5261 has a two-layer structure, a silicon nitride film and asilicon oxide film can be formed as a first insulating film and a secondinsulating film, respectively. In an example in the case where theinsulating film 5261 has a three-layer structure, a silicon oxide film,a silicon nitride film, and a silicon oxide film can be formed as afirst insulating film, a second insulating film, and a third insulatingfilm, respectively.

For each of the semiconductor layer 5262, the semiconductor layer 5303a, and the semiconductor layer 5303 b, for example, a non-single-crystalsemiconductor (e.g., amorphous silicon, polycrystalline silicon, ormicrocrystalline silicon), a single crystal semiconductor, a compoundsemiconductor or an oxide semiconductor (e.g., ZnO, InGaZnO, SiGe, GaAs,IZO, ITO, SnO, TiO, or AlZnSnO (AZTO)), an organic semiconductor, acarbon nanotube, or the like can be used.

Note that for example, the region 5262 a is an intrinsic region where animpurity is not added to the semiconductor layer 5262 and serves as achannel region. However, a slight amount of impurities can be added tothe region 5262 a. The concentration of the impurity added to the region5262 a is preferably lower than the concentration of an impurity addedto the region 5262 b, the region 5262 c, the region 5262 d, or theregion 5262 e. Each of the region 5262 b and the region 5262 d is aregion to which an impurity is added at low concentration and serves asan LDD (lightly doped drain) region. Note that the region 5262 b and theregion 5262 d can be eliminated. Each of the region 5262 c and theregion 5262 e is a region to which an impurity is added at highconcentration and serves as a source region or a drain region.

Note that the semiconductor layer 5303 b is a semiconductor layer towhich phosphorus or the like is added as an impurity element and hasn-type conductivity.

Note that in the case where an oxide semiconductor or a compoundsemiconductor is used for the semiconductor layer 5303 a, thesemiconductor layer 5303 b can be eliminated.

For each of the insulating layer 5263, the insulating layer 5302, andthe insulating layer 5356, a film containing oxygen or nitrogen, such assilicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), silicon nitride oxide (SiN_(x)O_(y)) (x>y), or alayered structure thereof can be used, for example.

As each of the conductive layer 5264, the conductive layer 5266, theconductive layer 5268, the conductive layer 5271, the conductive layer5301, the conductive layer 5304, the conductive layer 5306, theconductive layer 5308, the conductive layer 5357, and the conductivelayer 5359, for example, a conductive film having a single-layerstructure or a layered structure, or the like can be used. For example,for the conductive film, a single-layer film containing one elementselected from the group consisting of aluminum (Al), tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd), chromium(Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag), copper (Cu),manganese (Mn), cobalt (Co), niobium (Nb), silicon (Si), iron (Fe),palladium (Pd), carbon (C), scandium (Sc), zinc (Zn), phosphorus (P),boron (B), arsenic (As), gallium (Ga), indium (In), tin (Sn), oxygen(O), zirconium (Zr), and cerium (Ce); a compound containing one or moreelements selected from the above group; or the like can be used. Forexample, the compound is an alloy containing one or more elementsselected from the above group (e.g., an alloy material such as indiumtin oxide (ITO), indium zinc oxide (IZO), iridium tin oxide containingsilicon oxide (ITSO), zinc oxide (ZnO), tin oxide (SnO), cadmium tinoxide (CTO), aluminum-neodymium (Al—Nd), aluminum-tungsten (Al—Ta),aluminum-zirconium (Al—Zr), aluminum-titanium (Al—Ti), aluminum-cerium(Al—Ce), magnesium-silver (Mg—Ag), molybdenum-niobium (Mo—Nb),molybdenum-tungsten (Mo—W), or molybdenum-tantalum (Mo—Ta)); a compoundcontaining nitrogen and one or more elements selected from the abovegroup (e.g., a nitride film containing titanium nitride, tantalumnitride, molybdenum nitride, or the like); or a compound containingsilicon and one or more elements selected from the above group (e.g., asilicide film containing tungsten silicide, titanium silicide, nickelsilicide, aluminum silicon, or molybdenum silicon); or the like.Alternatively, a nanotube material such as a carbon nanotube, an organicnanotube, an inorganic nanotube, or a metal nanotube can be used.

Note that silicon (Si) can contain an n-type impurity (e.g., phosphorus)or a p-type impurity (e.g., boron).

Note that in the case where copper is used for the conductive layer, alayered structure is preferably used in order to improve adhesion.

Note that for a conductive layer which is in contact with an oxidesemiconductor or silicon, molybdenum or titanium is preferably used.

Note that by using an alloy material containing neodymium and aluminumfor the conductive layer, aluminum does not easily cause hillocks.

Note that in the case where a semiconductor material such as silicon isused for the conductive layer, the semiconductor material such assilicon can be formed at the same time as a semiconductor layer of atransistor.

Note that since ITO, IZO, ITSO, ZnO, Si, SnO, CTO, a carbon nanotube, orthe like has light-transmitting properties, such a material can be usedfor a portion through which light passes, such as a pixel electrode, acounter electrode, or a common electrode.

Note that by using a layered structure containing a low-resistancematerial (e.g., aluminum), wiring resistance can be lowered.

Note that by using a layered structure where a low heat-resistancematerial (e.g., aluminum) is interposed between high heat-resistancematerials (e.g., molybdenum, titanium, or neodymium), advantages of thelow heat-resistance material can be effectively utilized and heatresistance of a wiring, an electrode, or the like can be increased.

Note that a material whose properties are changed by reaction with adifferent material can be interposed between or covered with materialswhich do not easily react with the different material. For example, inthe case where ITO and aluminum are connected to each other, titanium,molybdenum, or an alloy of neodymium can be interposed between ITO andaluminum. For example, in the case where silicon and aluminum areconnected to each other, titanium, molybdenum, or an alloy of neodymiumcan be interposed between silicon and aluminum. Note that such amaterial can be used for a wiring, an electrode, a conductive layer, aconductive film, a terminal, a via, a plug, or the like.

For each of the insulating layer 5265, the insulating layer 5267, theinsulating layer 5269, the insulating layer 5305, and the insulatinglayer 5358, an insulating film having a single-layer structure or alayered structure, or the like can be used, for example. For example, asthe insulating film, an insulating film containing oxygen or nitrogen,such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)), siliconoxynitride (SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y))(x>y); a film containing carbon such as diamond-like carbon (DLC); anorganic material such as a siloxane resin, epoxy, polyimide, polyamide,polyvinyl phenol, benzocyclobutene, or acrylic; or the like can be used.

For the light-emitting layer 5270, an organic EL element, an inorganicEL element, or the like can be used, for example. For the organic ELelement, for example, a single-layer structure or a layered structure ofa hole injection layer formed using a hole injection material, a holetransport layer formed using a hole transport material, a light-emittinglayer formed using a light-emitting material, an electron transportlayer formed using an electron transport material, an electron injectionlayer formed using an electron injection material, or a layer in which aplurality of these materials are mixed can be used.

Note that an insulating layer which serves as an alignment film, aninsulating layer which serves as a protrusion portion, or the like canbe formed over the insulating layer 5305 and the conductive layer 5306.

Note that an insulating layer or the like which serves as a colorfilter, a black matrix, or a protrusion portion can be formed over theconductive layer 5308. An insulating layer which serves as an alignmentfilm can be formed below the conductive layer 5308.

Note that the insulating layer 5269, the light-emitting layer 5270, andthe conductive layer 5271 can be eliminated in the cross-sectionalstructure in FIG. 30A, and the liquid crystal layer 5307 and theconductive layer 5308 which are illustrated in FIG. 30B can be formedover the insulating layer 5267 and the conductive layer 5268.

Note that the liquid crystal layer 5307 and the conductive layer 5308can be eliminated in the cross-sectional structure in FIG. 30B, and theinsulating layer 5269, the light-emitting layer 5270, and the conductivelayer 5271 which are illustrated in FIG. 30A can be formed over theinsulating layer 5305 and the conductive layer 5306.

Note that in the cross-sectional structure in FIG. 30C, the insulatinglayer 5269, the light-emitting layer 5270, and the conductive layer 5271which are illustrated in FIG. 30A can be formed over the insulatinglayer 5358 and the conductive layer 5359. Alternatively, the liquidcrystal layer 5307 and the conductive layer 5308 which are illustratedin FIG. 30B can be foamed over the insulating layer 5267 and theconductive layer 5268.

The above is the description of one example of the structure of thetransistor in this embodiment. The transistor in this embodiment can beapplied to Embodiments 1 to 8. Specifically, in the case where anon-single-crystal semiconductor, a microcrystalline semiconductor, anorganic semiconductor, an oxide semiconductor, or the like is used asthe semiconductor layer in FIG. 30B, the transistor deteriorates in somecases. However, deterioration of the transistor in the semiconductordevice, the shift register, or the display device in Embodiments 1 to 8can be suppressed, which is advantageous.

Embodiment 10

In this embodiment, a layout view (hereinafter also referred to as a topview) of a shift register will be described. In this embodiment, forexample, a layout view of the shift register in Embodiment 4 will bedescribed. Note that a content described in this embodiment can beapplied to the semiconductor device, the shift register, or the displaydevice in Embodiments 1 to 9 in addition to the shift register inEmbodiment 4. Note that the layout view in this embodiment is oneexample and this embodiment is not limited to this example.

The layout view in this embodiment is described with reference to FIG.31 and FIG. 32. FIG. 31 illustrates one example of a layout view of partof the shift register. FIG. 32 illustrates a layout view of theflip-flop 401 _(—) i, for example.

A transistor, a wiring, and the like shown in FIG. 31 and FIG. 32 eachinclude a conductive layer 601, a semiconductor layer 602, a conductivelayer 603, a conductive layer 604, and a contact hole 605. However, thisembodiment is not limited to this example. A different conductive layer,insulating film, or a different contact hole can be newly formed. Forexample, a contact hole which connects the conductive layer 601 to theconductive layer 603 can be additionally provided.

The conductive layer 601 can include a portion which functions as a gateelectrode or a wiring. The semiconductor layer 602 can include a portionwhich functions as a semiconductor layer of the transistor. Theconductive layer 603 can include a portion which functions as a wiringor a source or drain. The conductive layer 604 can include a portionwhich functions as a transparent electrode, a pixel electrode, or awiring. The contact hole 605 has a function of connecting the conductivelayer 601 and the conductive layer 604 with each other or connecting theconductive layer 603 and the conductive layer 604 with each other.

In the example in FIG. 31, the wiring 412 includes an opening portion611 and the wiring 413 includes an opening portion 612. In this manner,since the wiring 412 and the wiring 413 include the opening portions,parasitic capacitance can be reduced. Alternatively, the breakdown ofthe transistor due to electrostatic discharge can be suppressed.However, this embodiment is not limited to this example. As in the caseof the wiring 416, the opening portion 611 or the opening portion 612can be eliminated. Alternatively, like in the case of the wiring 412 orthe wiring 413, an opening portion can be provided for the wiring 416.

In the example in FIG. 31, by provision of an opening portion in part ofan intersection portion of the wiring 412 or the wiring 413 and adifferent wiring, the cross-over capacitance of the wiring can bereduced. Accordingly, reduction in noise or reduction in delay ordistortion of a signal can be achieved.

In the example in FIG. 31, the conductive layer 604 is formed over partof the conductive layer 603 which the wiring 416 includes. In addition,the conductive layer 604 is connected to the conductive layer 603through the contact hole 605. In this manner, since wiring resistancecan be made low, suppression of voltage drop or reduction in delay ordistortion of a signal can be achieved. However, this embodiment is notlimited to this example. The conductive layer 604 and the contact hole605 can be eliminated. Alternatively, like in the case of the wiring416, the conductive layer 604 can be formed over part of the conductivelayer 603 which the wiring 412 or the wiring 413 includes so that theconductive layer 604 can be connected to the conductive layer 603.

Here, in the example in FIG. 31, the width of the wiring 412, the widthof the wiring 413, and the width of the wiring 416 are referred to aswiring width 621, wiring width 622, and width 623, respectively. Then,the width of the opening portion 611, the length of the opening portion611, the width of the opening portion 612, and the length of the openingportion 612 are referred to as width 624, length 625, width 626, andlength 627, respectively.

Note that signals input to the wiring 412 and the wiring 413 areinverted signals of each other in many cases. Therefore, the wiringresistance or the parasitic capacitance of the wiring 412 is preferablyset so as to be approximately equal to that of the wiring 413.Accordingly, the wiring 412 preferably includes a portion whose width isapproximately equal to the wiring width 622. Alternatively, the openingportion 611 preferably includes a portion whose width is approximatelyequal to the width 626 of the opening portion 612 or a portion whoselength is approximately equal to the length 627 of the opening portion612. However, this embodiment is not limited to this example. The wiringwidth 621, the wiring width 622, the width 624 of the opening portion611, the length 625 of the opening portion 611, or the length 627 of theopening portion 612 can have a variety of values. For example, when thecross-over capacitance of the wiring 412 and a different wiring ishigher than that of the wiring 413 and a different wiring, delay ordistortion of signals input to the wiring 412 and the wiring 413 can bemade approximately the same by decreasing the wiring resistance of thewiring 412. Therefore, the wiring 412 can include a portion which iswider than the wiring width 622. Alternatively, the opening portion 611can include a portion which is narrower than the width 626 of theopening portion 612. Alternatively, the opening portion 611 can includea portion which is shorter than the length 627 of the opening portion612. On the other hand, when the cross-over capacitance of the wiring412 and a different wiring is lower than that of the wiring 413 and adifferent wiring, the wiring 412 can include a portion which is narrowerthan the wiring width 622. Alternatively, the opening portion 611 caninclude a portion which is wider than the width 626 of the openingportion 612. Alternatively, the opening portion 611 can include aportion which is longer than the length 627 of the opening portion 612.

In the case where the wiring 416 does not include the opening portion,the wiring 416 preferably includes a portion which is narrower than thewiring width 621 or the wiring width 622. This is because the wiring 416does not include an opening portion, and the wiring resistance of thewiring 416 is low. However, this embodiment is not limited to thisexample. The wiring 416 can include a portion which is wider than thewiring width 621 or the wiring width 622.

In the example in FIG. 32, in the transistor 101, the transistor 102,the transistor 103, the transistor 201, the transistor 202, thetransistor 203, the transistor 204, the transistor 301, the transistor302, the transistor 303, the transistor 304, and/or the transistor 305,an area where the conductive layers 601 and the conductive layers 603 ofthe second terminals overlap with each other is preferably smaller thanan area where the conductive layers 601 and the conductive layers 603 ofthe first terminals overlap with each other. In this manner, reductionin noise of the gate of the transistor 101 or the wiring 401 _(—) i canbe achieved. Alternatively, since concentration of an electric field onthe second terminal can be suppressed, deterioration of the transistoror the breakdown of the transistor can be suppressed.

The above is the description of one example of the layout view of theshift register. However, as described above, the layout view of thisembodiment is one example and this embodiment is not limited to thisexample.

Note that the semiconductor layer 602 can be provided for a portion inwhich the conductive layer 601 and the conductive layer 603 overlap witheach other. Accordingly, the parasitic capacitance between theconductive layer 601 and the conductive layer 603 can be reduced,whereby reduction in noise can be achieved. Because of a similar reason,the semiconductor layer 602 or the conductive layer 603 can be providedfor a portion in which the conductive layer 601 and the conductive layer604 overlap with each other.

Note that the conductive layer 604 can be formed over part of theconductive layer 601 and can be connected to the conductive layer 601through the contact hole 605. Accordingly, wiring resistance can bereduced. Alternatively, the conductive layer 603 and the conductivelayer 604 can be formed over part of the conductive layer 601, so thatthe conductive layer 601 is connected to the conductive layer 604through the contact hole 605 and the conductive layer 603 can beconnected to the conductive layer 604 through the different contact hole605. In this manner, the wiring resistance can be further reduced.

Note that the conductive layer 604 can be formed over part of theconductive layer 603, so that the conductive layer 603 can be connectedto the conductive layer 604 through the contact hole 605. In thismanner, wiring resistance can be reduced.

Note that the conductive layer 601 or the conductive layer 603 can beformed under part of the conductive layer 604, so that the conductivelayer 604 can be connected to the conductive layer 601 or the conductivelayer 603 through the contact hole 605. In this manner, wiringresistance can be reduced.

Note that as described in Embodiment 1, the parasitic capacitancebetween the gate and the second terminal of the transistor 101 can behigher than the parasitic capacitance between the gate and the firstterminal of the transistor 101. As shown in FIG. 32, the width of theconductive layer 603 which can function as a first electrode of thetransistor 101 is referred to as width 631 and the width of theconductive layer 603 which can function as a second electrode of thetransistor 101 is referred to as width 632. In addition, the width 631can be larger than the width 632. In this manner, as described inEmbodiment 1, the parasitic capacitance between the gate and the secondterminal of the transistor 101 can be higher than the parasiticcapacitance between the gate and the first terminal of the transistor101. However, this embodiment is not limited to this example.

Embodiment 11

In this embodiment, examples of electronic devices are described.

FIGS. 33A to 33H and FIGS. 34A to 34D illustrate electronic devices.These electronic devices can include a housing 5000, a display portion5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including apower switch or an operation switch), a connection terminal 5006, asensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 5008, and the like.

FIG. 33A illustrates a mobile computer, which can include a switch 5009,an infrared port 5010, and the like in addition to the above objects.FIG. 33B illustrates a portable image reproducing device provided with amemory medium (e.g., a DVD regenerating device), which can include asecond display portion 5002, a memory medium reading portion 5011, andthe like in addition to the above objects. FIG. 33C illustrates agoggle-type display, which can include the second display portion 5002,a support portion 5012, an earphone 5013, and the like in addition tothe above objects. FIG. 33D illustrates a portable game machine, whichcan include the memory medium reading portion 5011 and the like inaddition to the above objects. FIG. 33E illustrates a projector, whichcan include a light source 5033, a projector lens 5034, and the like inaddition to the above objects. FIG. 33F illustrates a portable gamemachine, which can include the second display portion 5002, the memorymedium reading portion 5011, and the like in addition to the aboveobjects. FIG. 33G illustrates a television receiver, which can include atuner, an image processing portion, and the like in addition to theabove objects. FIG. 33H illustrates a portable television receiver,which can include a charger 5017 capable of transmitting and receivingsignals and the like in addition to the above objects. FIG. 34Aillustrates a display, which can include a support base 5018 and thelike in addition to the above objects. FIG. 34B illustrates a camera,which can include an external connecting port 5019, a shutter button5015, an image receiving portion 5016, and the like in addition to theabove objects. FIG. 34C illustrates a computer, which can include apointing device 5020, the external connecting port 5019, a reader/writer5021, and the like in addition to the above objects. FIG. 34Dillustrates a mobile phone, which can include an antenna 5014, a tunerof one-segment partial reception service for mobile phones and mobileterminals, and the like in addition to the above objects.

The electronic devices illustrated in FIGS. 33A to 33H and FIGS. 34A to34D can have a variety of functions, for example, a function ofdisplaying a lot of information (e.g., a still image, a moving image,and a text image) on a display portion; a touch panel function; afunction of displaying a calendar, date, time, and the like; a functionof controlling processing with a lot of software (programs); a wirelesscommunication function; a function of being connected to a variety ofcomputer networks with a wireless communication function; a function oftransmitting and receiving a lot of data with a wireless communicationfunction; and a function of reading a program or data stored in a memorymedium and displaying the program or data on a display portion. Further,the electronic device including a plurality of display portions can havea function of displaying image information mainly on one display portionwhile displaying text information on another display portion, a functionof displaying a three-dimensional image by displaying images whereparallax is considered on a plurality of display portions, or the like.Furthermore, the electronic device including an image receiving portioncan have a function of photographing a still image, a function ofphotographing a moving image, a function of automatically or manuallycorrecting a photographed image, a function of storing a photographedimage in a memory medium (an external memory medium or a memory mediumincorporated in the camera), a function of displaying a photographedimage on the display portion, or the like. Note that functions which canbe provided for the electronic devices illustrated in FIGS. 33A to 33Hand FIGS. 34A to 34D are not limited to them, and the electronic devicescan have a variety of functions.

The electronic devices described in this embodiment each include adisplay portion for displaying some kind of information. By acombination of the electronic device in this embodiment and thesemiconductor device, the shift register, or the display device inEmbodiments 1 to 9, improved reliability, improved yield, costreduction, a large display portion, a display portion with highdefinition, or the like can be achieved.

Next, applications of semiconductor devices are described.

FIG. 34E illustrates an example in which a semiconductor device isincorporated in a building structure. FIG. 34E illustrates a housing5022, a display portion 5023, a remote controller 5024 which is anoperation portion, a speaker 5025, and the like. The semiconductordevice is incorporated in the building structure as a wall-hanging typeand can be provided without requiring a large space.

FIG. 34F illustrates another example in which a semiconductor device isincorporated in a building structure. A display panel 5026 isincorporated in a prefabricated bath unit 5027, so that a bather canview the display panel 5026.

Note that although this embodiment describes the wall and theprefabricated bath are given as examples of the building structures,this embodiment is not limited to them. The semiconductor devices can beprovided in a variety of building structures.

Next, examples in which semiconductor devices are incorporated in movingobjects are described.

FIG. 34G illustrates an example in which a semiconductor device isincorporated in a car. A display panel 5028 is incorporated in a carbody 5029 of the car and can display information related to theoperation of the car or information input from inside or outside of thecar on demand. Note that the display panel 5028 may have a navigationfunction.

FIG. 34H illustrates an example in which a semiconductor device isincorporated in a passenger airplane. FIG. 34H illustrates a usagepattern when a display panel 5031 is provided for a ceiling 5030 above aseat of the passenger airplane. The display panel 5031 is incorporatedin the ceiling 5030 through a hinge portion 5032, and a passenger canview the display panel 5031 by stretching of the hinge portion 5032. Thedisplay panel 5031 has a function of displaying information by theoperation of the passenger.

Note that although bodies of a car and an airplane are illustrated asexamples of moving objects in this embodiment, this embodiment is notlimited to them. The semiconductor device can be provided for a varietyof objects such as two-wheeled vehicles, four-wheeled vehicles(including cars, buses, and the like), trains (including monorails,railroads, and the like), and vessels.

This application is based on Japanese Patent Application serial no.2009-007419 filed with Japan Patent Office on Jan. 16, 2009, the entirecontents of which are hereby incorporated by reference.

1. A liquid crystal display device comprising: a driver circuit, thedriver circuit including: a first transistor; a second transistor; athird transistor; a first circuit; and a second circuit; and a pixelincluding a liquid crystal element, wherein a first terminal of thefirst transistor is electrically connected to a second wiring, wherein asecond terminal of the first transistor is electrically connected to afirst wiring, and wherein a gate of the first transistor is electricallyconnected to the second circuit and a first terminal of the thirdtransistor, wherein a first terminal of the second transistor iselectrically connected to the first wiring, wherein a second terminal ofthe second transistor is electrically connected to a sixth wiring,wherein a gate of the second transistor is electrically connected thefirst circuit and a gate of the third transistor, wherein a secondterminal of the third transistor is electrically connected to the sixthwiring, wherein the first circuit is electrically connected to the firstwiring, the second wiring, and the sixth wiring, and wherein the secondcircuit is electrically connected to a third wiring, a fourth wiring, afifth wiring, and the sixth wiring.
 2. The liquid crystal display deviceaccording to claim 1, wherein the second wiring is a signal line or aclock signal line, wherein the first wiring is a signal line, a gateline, a scan line, or an output signal line, wherein the sixth wiring isa power supply line or a ground line, wherein a third wiring is a signalline or a clock signal line, wherein a fourth wiring is a signal line,and wherein a fifth wiring is a signal line.
 3. The liquid crystaldisplay device according to claim 1, wherein the first transistor is abootstrap transistor for controlling timing when a signal of the secondwiring is supplied to the first wiring in accordance with a potential ofthe gate of the first transistor.
 4. The liquid crystal display deviceaccording to claim 1, wherein the second transistor is a switch forcontrolling an electrical conduction state of the sixth wiring and thefirst wiring in accordance with an output signal from the first circuitor a potential of the gate of the second transistor.
 5. The liquidcrystal display device according to claim 1, wherein the thirdtransistor is a switch for controlling an electrical conduction state ofthe sixth wiring and the gate of the first transistor in accordance withan output signal from the first circuit.
 6. The liquid crystal displaydevice according to claim 1, wherein the first circuit is a circuit forincreasing, decreasing, and maintaining a potential of the gate of thesecond transistor or a controlling circuit for making the gate of thesecond transistor go into a floating state, by controlling timing when avoltage of the sixth wiring is supplied to the gate of the secondtransistor in accordance with a signal of the first wiring or a signalof the second wiring.
 7. The liquid crystal display device according toclaim 1, wherein the second circuit is a circuit for increasing,decreasing, and maintaining a potential of the gate of the firsttransistor or a controlling circuit for making the gate of the firsttransistor go into a floating state, by controlling timing when a signalsupplied to the fourth wiring or a voltage of the sixth wiring issupplied to the gate of the first transistor in accordance with a signalsupplied to the third wiring, a signal supplied to the fourth wiring, ora signal supplied to the fifth wiring.
 8. The liquid crystal displaydevice according to claim 1, wherein the first circuit includes: afourth transistor; a fifth transistor; a sixth transistor; and a seventhtransistor, wherein a first terminal of the fourth transistor iselectrically connected to the second wiring, wherein a second terminalof the fourth transistor is electrically connected to the gate of thesecond transistor, wherein a first terminal of the fifth transistor iselectrically connected to the sixth wiring, wherein a second terminal ofthe fifth transistor is electrically connected to the gate of the secondtransistor, wherein a gate of the fifth transistor is electricallyconnected to the first wiring, wherein a first terminal of the sixthtransistor is electrically connected to the second wiring, wherein asecond terminal of the sixth transistor is electrically connected to thegate of the fourth transistor, wherein a gate of the sixth transistor iselectrically connected to the second wiring, and wherein a firstterminal of the seventh transistor is electrically connected to thesixth wiring, wherein a second terminal of the seventh transistor iselectrically connected to the gate of the fourth transistor, and whereina gate of the seventh transistor is electrically connected to the firstwiring.
 9. The liquid crystal display device according to claim 1,wherein the second circuit includes: an eighth transistor; a ninthtransistor; a tenth transistor; an eleventh transistor; and a twelfthtransistor, wherein a first terminal of the eighth transistor iselectrically connected to the fourth wiring, wherein a second terminalof the eighth transistor is electrically connected to the gate of thefirst transistor, wherein a gate of the eighth transistor iselectrically connected to the third wiring, wherein a first terminal ofthe ninth transistor is electrically connected to the gate of the firsttransistor, wherein a second terminal of the ninth transistor iselectrically connected to the fourth wiring, wherein a gate of the ninthtransistor is electrically connected to the fourth wiring, wherein afirst terminal of the tenth transistor is electrically connected to thegate of the first transistor, wherein a second terminal of the tenthtransistor is electrically connected to the sixth wiring, wherein a gateof the tenth transistor is electrically connected to the fifth wiring,wherein a first terminal of the eleventh transistor is electricallyconnected to the first wiring, wherein a second terminal of the eleventhtransistor is electrically connected to the sixth wiring, wherein a gateof the eleventh transistor is electrically connected to the fifthwiring, and wherein a first terminal of the twelfth transistor iselectrically connected to the first wiring, wherein a second terminal ofthe twelfth transistor is electrically connected to the sixth wiring,and wherein a gate of the twelfth transistor is electrically connectedto the third wiring.
 10. The liquid crystal display device according toclaim 1, wherein the driver circuit is formed over the same substrate asthe pixel.
 11. The liquid crystal display device according to claim 1,wherein the channel width of the first transistor is larger than that ofthe second transistor and that of the third transistor.
 12. Anelectronic device including the liquid crystal display device accordingto claim 1 and an operation switch.
 13. A liquid crystal display devicecomprising: a driver circuit, the driver circuit including: a firsttransistor; a second transistor; a first circuit; and a second circuit;and a pixel including a liquid crystal element, wherein a first terminalof the first transistor is electrically connected to a second wiring,wherein a second terminal of the first transistor is electricallyconnected to a first wiring, wherein a gate of the first transistor iselectrically connected to the second circuit, wherein a first terminalof the second transistor is electrically connected to the first wiring,wherein a second terminal of the second transistor is electricallyconnected to a sixth wiring, wherein a gate of the second transistor iselectrically connected the first circuit, wherein the first circuit iselectrically connected to the first wiring, the second wiring, and thesixth wiring, and wherein the second circuit is electrically connectedto a third wiring, a fourth wiring, a fifth wiring, and the sixthwiring.
 14. The liquid crystal display device according to claim 13,wherein the second wiring is a signal line or a clock signal line,wherein the first wiring is a signal line, a gate line, a scan line, oran output signal line, wherein the sixth wiring is a power supply lineor a ground line, wherein a third wiring is a signal line or a clocksignal line, wherein a fourth wiring is a signal line, and wherein afifth wiring is a signal line.
 15. The liquid crystal display deviceaccording to claim 13, wherein the first transistor is a bootstraptransistor for controlling timing when a signal of the second wiring issupplied to the first wiring in accordance with a potential of the gateof the first transistor.
 16. The liquid crystal display device accordingto claim 13, wherein the second transistor is a switch for controllingan electrical conduction state of the sixth wiring and the first wiringin accordance with an output signal from the first circuit or apotential of the gate of the second transistor.
 17. The liquid crystaldisplay device according to claim 13, wherein the first circuit is acircuit for increasing, decreasing, and maintaining a potential of thegate of the second transistor or a controlling circuit for making thegate of the second transistor go into a floating state, by controllingtiming when a voltage of the sixth wiring is supplied to the gate of thesecond transistor in accordance with a signal of the first wiring or asignal of the second wiring.
 18. The liquid crystal display deviceaccording to claim 13, wherein the second circuit is a circuit forincreasing, decreasing, and maintaining a potential of the gate of thefirst transistor or a controlling circuit for making the gate of thefirst transistor go into a floating state, by controlling timing when asignal supplied to the fourth wiring or a voltage of the sixth wiring issupplied to the gate of the first transistor in accordance with a signalsupplied to the third wiring, a signal supplied to the fourth wiring, ora signal supplied to the fifth wiring.
 19. The liquid crystal displaydevice according to claim 13, wherein the first circuit includes: afourth transistor; a fifth transistor; a sixth transistor; and a seventhtransistor, wherein a first terminal of the fourth transistor iselectrically connected to the second wiring, wherein a second terminalof the fourth transistor is electrically connected to the gate of thesecond transistor, wherein a first terminal of the fifth transistor iselectrically connected to the sixth wiring, wherein a second terminal ofthe fifth transistor is electrically connected to the gate of the secondtransistor, wherein a gate of the fifth transistor is electricallyconnected to the first wiring, wherein a first terminal of the sixthtransistor is electrically connected to the second wiring, wherein asecond terminal of the sixth transistor is electrically connected to thegate of the fourth transistor, wherein a gate of the sixth transistor iselectrically connected to the second wiring, wherein a first terminal ofthe seventh transistor is electrically connected to the sixth wiring,wherein a second terminal of the seventh transistor is electricallyconnected to the gate of the fourth transistor, and wherein a gate ofthe seventh transistor is electrically connected to the first wiring.20. The liquid crystal display device according to claim 13, wherein thesecond circuit includes: an eighth transistor; a ninth transistor; atenth transistor; an eleventh transistor; and a twelfth transistor,wherein a first terminal of the eighth transistor is electricallyconnected to the fourth wiring, wherein a second terminal of the eighthtransistor is electrically connected to the gate of the firsttransistor, wherein a gate of the eighth transistor is electricallyconnected to the third wiring, wherein a first terminal of the ninthtransistor is electrically connected to the gate of the firsttransistor, wherein a second terminal of the ninth transistor iselectrically connected to the fourth wiring, wherein a gate of the ninthtransistor is electrically connected to the fourth wiring, wherein afirst terminal of the tenth transistor is electrically connected to thegate of the first transistor, wherein a second terminal of the tenthtransistor is electrically connected to the sixth wiring, wherein a gateof the tenth transistor is electrically connected to the fifth wiring,wherein a first terminal of the eleventh transistor is electricallyconnected to the first wiring, wherein a second terminal of the eleventhtransistor is electrically connected to the sixth wiring, wherein a gateof the eleventh transistor is electrically connected to the fifthwiring, and wherein a first terminal of the twelfth transistor iselectrically connected to the first wiring, wherein a second terminal ofthe twelfth transistor is electrically connected to the sixth wiring,and wherein a gate of the twelfth transistor is electrically connectedto the third wiring.
 21. The liquid crystal display device according toclaim 13, wherein the driver circuit is formed over the same substrateas the pixel.
 22. The liquid crystal display device according to claim13, wherein the channel width of the first transistor is larger thanthat of the second transistor.
 23. An electronic device including theliquid crystal display device according to claim 13 and an operationswitch.